ZOOLOGY 


GALLOWAY 


BY  THE  SAME  AUTHOR 


ELEMENTARY  ZOOLOGY 

A  Text-book  for  Secondary 
Educational  Institutions 

160    ILLUSTRATIONS 
i2mo,  xx+4i8  pages.     Cloth,  $1.25 


BLAKISTON'S     SCIENCE     SERIES 

ZOOLOGY 

A  TEXT-BOOK  FOR 
UNIVERSITIES,  COLLEGES 
AND    NORMAL   SCHOOLS 


BY 

THOMAS  WALTON  GALLOWAY,  PH.  D.,  Lrnr.  D. 

'ROFESSOR   OF   ZOOLOGY,  BELOIT   COLLEGE,  BELOIT,   WISCONSIN;  FORMERLY  PROFESSOR 
OF  BIOLOGY  IN   THE  JAMES  MILLIKIN   UNIVERSITY,   DECATUR,   ILLINOIS 


THIRD  EDITION,  REVISED 
WITH  255  ILLUSTRATIONS 


PHILADELPHIA 

P.   BLAKISTON'S   SON   &   CO. 

1012  WALNUT   STREET 


COPYRIGHT,  1915,  BY  P.  BLAKISTON'S  SON  &  Co. 


MAPI.K    PRESS    YORK    I»A 


PREFACE  TO  THE  THIRD  EDITION 


The  last  twenty  years  have  been  very  productive  of  text- 
books of  Zoology,  both  for  secondary  schools  and  for  colleges. 
Many  of  these  are  admirable  from  the  point  of  view  of  the'science 
itself.  It  is  increasingly  apparent,  however,  that  the  right 
text-book  of  Zoology,  as  of  every  other  subject,  is  primarily  a 
matter  of  psychology.  The  prime  object  of  all  teaching  is  so 
to  use  the  subject  as  to  produce  the  best  possible  result  in  the 
personality  of  the  pupil.  This  means  that  the  mental  structure 
and  functioning  of  the  pupils  are  even  more  important  than  the 
subject  matter  in  conditioning  the  presentation  of  any  subject. 
Teachableness  is  the  first  prerequisite  in  any  text-book  in  any 
subject  whatsoever.  Some  of  the  best  zoologies  we  have  from 
the  point  of  view  of  the  science  of  Zoology  are  pedagogically 
impracticable. 

It  is  the  purpose  of  the  present  book,  however  much  it  may 
fall  short  of  its  aim,  to  subordinate  certain  zoological  prepos- 
sessions to  the  mental  requirements  of  freshmen.  The  method 
here  adopted  has  been  used  successfully  by  the  writer  in  a  first 
course,  for  more  than  fifteen  years.  It  has  secured  good  interest 
and  fine  spirit.  The  following  are  some  of  the  principles  that 
have  guided  him  in  the  selection  and  arrangement  of  the  mate- 
rial in  the  present  volume: 

i.  The  work  done  in  a  first  course  is  primarily  for  pupils 
who  do  not  take  a  second  course.  This  first  course  should  be 
handled,  therefore,  as  a  life-training  rather  than  as  satisfying 
a  college  requirement.  It  should  seek  to  accomplish  these, 
among  other,  things: 

The  production  and  conservation  of  a  vital  interest  in 
animals;  an  appreciation  of  the  human  values  of  animals; 
the  encouragement  of  the  attitude  of  raising  and  solving 
problems  concerning  animals;  some  ability  to  use  the 
library,  the  field,  and  the  laboratory  in  individual  pursuit 
of  these  interests;  the  ability  to  sustain  interest  in  these 


VI  PREFACE    TO    THE    THIRD    EDITION 

problems  through  considerable  periods ;  a  sense  of  the  way 
in  which  organisms  respond  to  the  environing  conditions ; 
an  elementary  conception  of  development  and  of  the  evolu- 
tionary series  of  animals ;  some  experience  in  classification 
of  organisms — theoretical  and  practical;  a  conception  of 
the  place  of  man  in  the  biological  series,  along  with  the 
conviction  that  this  does  not  invalidate,  but  rather 
heightens,  the  meaning  of  all  the  higher  human  qualities. 

2.  The  first  thing  to  be  sought  therefore  is  a  thorough-going 
appreciation  on  the  part  of  the  student  of  the  attractiveness, 
the  scope,  and  importance  of  animals  and  their  activities. 

3.  A  first  course  should  really  be  a  foundation  course,  and 
as  such  should  give  the  student  a  broad  and  catholic  view  of 
the  whole  subject.     It  should  utilize  all  the  main  departments 
of  Zoology,  because  each  department  contains  matter  which 
should  be  familiar  to  all  persons  of  ordinary  education.     Further- 
more, the  departments  of  morphology,  physiology,  ecology,  dis- 
tribution, and  classification  furnish  exercises  which  have  dis- 
tinct, and  yet  complementary,  pedagogical  value.     Any  single 
phase  of  the  subject,  however  important  or  interesting,  gives 
a  false  and  therefore  an  unscientific  view  of  the  wonderful  science 
of  Zoology,  unless  it  is  supplemented  by  the  others.     Therefore 
a  book,  if  it  is  to  serve  the  pedagogical  needs  of  beginners,  should 
contain  fairly  representative  matter  from  all  the  main  depart- 
ments of  the  science;  and  it  should  at  the  same  time  provide 
both  for  the  descriptive  work  and  for  the  practical  work  in  the 
field  and  laboratory. 

4.  Laboratory  work  and  field  work  are  essential,  both  to 
proper  interest  and  to  proper  results,  and  should  not  be  merely 
illustrative  of  text  or  lecture  work,  but  as  far  as  possible  should 
be  the  foundation  and  point  of  departure  of  the  lectures  and 
the  text.     No  instrumentality  open  to  the  teacher  is  better  than 
the  laboratory  as  a  means  of  securing  real  interest  and  mental 
growth  for  the  pupils.     However,  in  order  to  attain  this  end 
it  is  essential  that  this  work  shall  really  be  vitally  done.     It  is 
not  enough  that  a  pupil  be  induced  to  observe  and  to  record 
his   observations.     The   pupil's   mind   should   always   be  en- 
couraged to  "follow  through"  to  whatever  response  in  the  way 
of  conclusion  or  explanation  seems  sound  in  the  light  of  his 
knowledge  at  the  time.     It  is  much  more  important  that  these 


PREFACE    TO    THE    THIRD    EDITION  Vll 

inner  reactions  be  allowed,  be  complete,  and  be  the  student's 
own,  than  that  they  be  rigorously  right.  It  is  easy  enough  to 
add  new  facts  in  order  both  to  teach  the  pupil  that  conclusions 
are  liable  to  be  wrong,  and  to  force  his  conclusion  closer  to  that 
moving  equilibrium  which  we  call  truth.  This  process  is  much 
more  easy  than  to  re-galvanize  the  soul  whose  interest  and 
ability  to  reach  conclusions  have  been  aborted  by  a  continual 
denial  of  the  right.  Much  of  the  failure  in  our  laboratory  work 
is  due  to  this  "taxation  without  representation"  in  respect  to 
personality. 

5.  On  the  other  hand  it  is  equally  important  that  the  work 
shall  not  be  confined  to  the  field  and  the  laboratory.     "There 
are  many  things  in  the  infinite  concourse  of  particulars  which 
we  cannot  afford  to  verify  by  experiment."     The  chief  end  of 
laboratory  work  is  gained  for  the  elementary  student  when  he 
comes  to  appreciate  the  method  and  spirit  by  which  sound  in- 
vestigation proceeds,  has  acquired  enough  technical  skill  to  fol- 
low elementary  investigation  on  his  own  behalf,  and  has  learned 
how  to  appreciate,  and  if  necessary  to  verify,  the  statements  of 
others.     It  is  as  easy  to  waste  time  in  the  laboratory  as  in  reading 
textbooks. 

6.  The  time  in  an  elementary  course  should  be  about  equally 
apportioned  (i)  to  laboratory  work  (chiefly  in  physiology  and 
in  the  larger  problems  of  morphology  rather  than  in  minute 
dissection);  (2)  to  field  observation  on  physiology,  life  histories, 
and  the  simpler  problems  of  distribution,   classification,   and 
life  relations;  (3)  to  the  body  of  the  descriptive  text;  and  (4)  to 
classes  of  questions  demanding  reference  to  classical  zoological 
authorities. 

It  is  a  great  mistake  not  to  impress  upon  the  student  the 
immense  amount  of  work  already  done  and  to  heighten  his  re- 
spect for  the  library  as  one  of  his  sources  of  information  and 
interest. 

7.  The  matter  of  greater  native  interest  should  underlie  and 
sustain  that  of  less.     It  should  not,  however,  exclude  or  efface 
the  latter.     The  most  interesting  is  often  the  least  important. 

8.  Certain  of  the  general  facts  and  principles  which  the 
beginner  cannot  be  expected  to  discover  for  himself  should  be 


Vlll  PREFACE   TO   THE   THIRD   EDITION 

presented  early,  in  order  to  give  the  student  a  skeleton — or 
dimensions,  so  to  speak — in  which  he  shall  later  insert  the  par- 
ticulars which  he  discovers.  He  must  have  this  in  order  to 
unify  his  own  results  in  the  brief  time  at  his  disposal.  The 
lack  of  this  unifying  result  is  the  ground  of  the  just  complaint 
concerning  much  of  the  unorganized  and  unrelated  laboratory 
instruction  in  the  secondary  schools  and  early  college  classes. 

9.  While  it  is  necessary  to  bring  our  materials  from  various 
departments  of  Zoology  and  is  desirable  that  the  student  should 
be  able  to  recognize  whether  a  given  problem  is  primarily  one 
of  structure  or  function  or  relation,  the  total  result  of  an  ele- 
mentary course  of  Zoology  should  be  a  sense  of  unity,  of  con- 
tinuity, and  of  interdependence.     The  final  view  of  the  student 
should  be  of  life  and  organic  progress,  and  not  of  a  disjointed 
science,  dissected  in  the  house  of  its  friends. 

10.  The  teacher  should  have  some  latitude  in  the  choice  of 
matter  and  emphasis,  in  order  that  both  may  be  properly  suited 
to  his  equipment  and  locality.     It  should  be  impossible  for  the 
teacher  or  the  class  to  use  a  text-book  in  a  slavish,  or  parasitic 
fashion.     Therefore  a   text-book  should  contain  and  suggest 
much  more  than  one  teacher  or  one  class  can  use  in  the  time 
allowed.     This  not  only  gives  the  teacher  a  chance  (and  makes 
it  necessary  for  him)  to  mould  his  own  course,  but  causes  the 
student  -to  realize  that  he  is  a  mere  beginner  when  he  has  com- 
pleted his  first  course. 

In  attempting  to  apply  these  principles  to  the  present  book 
the  author  has  made  use  of  the  following  devices : 

i.  The  book  is  divided  into  three  portions: — (i)  a  general 
part  dealing  largely  with  broad  biological  problems  and  princi- 
ples, which  constitute  the  foundations  of  the  science  and  are 
felt  to  be  for  the  most  part,  beyond  even  the  verification  of  the 
elementary  student  (Chapters  I-VIII) ;  (2)  a  special  part  (Chap- 
ters IX-XXV),  in  which  the  various  principal  phyla  of  animals 
are  taken  up  in  succession,  beginning  with  the  lowest.  The 
purpose  has  been  to  make  this  part  particularly  illustrative  of 
the  principles  laid  down  in  the  general  portion;  and  (3)  a  group 
of  synthetic  chapters  (XXVI-XXIX)  to  induce  the  student  to 
gather  up  the  details  of  his  course  by  a  new  reorganization  of 
the  materials. 


PREFACE    TO    THE    THIRD    EDITION  x 

2.  Each  chapter  of  the  general  part  contains  the  following 
elements: — (i)   the  general  statement  of  principles  or  facts; 
(2)  interspersed  with  this  are  such  practical  exercises  for  labo- 
ratory, field,  or  library,  as  have  been  found  practicable  for 
elementary  classes.     These  are  intended  to  compensate  for  the 
enforced  brevity  and  abstractness  of  definitions  and  description, 
by  causing  the  student  to  find  concrete  illustration  of  the  princi- 
ples; (3)  an  analytic  summary  of  the  most  important  general 
truths  of  the  chapter  in  outline,  at  the  close  of  the  chapter; 
and  finally  (4),  a  list  of  supplementary  topics  for  individual 
laboratory  or  library  investigation  and  report.     These  supple- 
ment and  illustrate  the  text,  and  enrich  the  review  by  introduc- 
ing a  new  viewpoint  and  new  matter.     It  is  not  intended  that 
all  of  these  topics  shall  be  demanded  of  the  whole  class.     The 
writer  has  got  best  results  in  interest  and  knowledge  by  allow- 
ing each  member  of  the  class  to  select  some  topic  in  which  he  is 
interested  and  to  make  a  brief  report  of  his  investigation  before 
the  whole  group. 

3.  In  the  chapters  of  the  special  part  each  phylum  is  intro- 
duced by  field  and  laboratory  work  on  some  representatives 
taken  as  types.     This  is  purposely  made  brief  and  suggestive 
in  order  to  stimulate  the  teacher  and  class  to  build  up  their 
own  detailed  program.     This  is  followed,  corrected,  and  en- 
larged by  a  brief  discussion  of  the  typical  condition  of  the  organs 
and  functions  in  the  group  as  a  whole.     This  serves  to  unify  the 
isolated  and  local  observations  of  the  student.     Next  follows 
a  brief  statement  of  the  most  important  facts  of  classification, 
together  with  ecological  and  economic  suggestions.     Finally, 
each  chapter  concludes  with  a  list  of  supplementary  questions 
calling  for  field,  laboratory,  and  library  work  in  review,  and  as 
a  brief  view  of  new  material. 

4.  The  figures  are  carefully  selected, — the  majority  of  them 
being  specially  made  for  this  book.     With  each  figure  of  special 
moment  is  a  brief  list  of  queries  designed  to  assist  the  student 
in  the  study  of  the  figure.     It  is  a  common  complaint  among 
teachers  that  it  is  difficult  to  get  students  to  appreciate  and  to 
use  illustrations  intelligently,  and  to  relate  them  to  the  text. 
A  sane  emphasis  on  these  questions  will  solve  this  problem. 


X  PREFACE    TO    THE    THIRD    EDITION 

5.  The  concluding  chapter  consists  of  practical  questions 
and  special  exercises  which  necessitate  a  review  by  the  studen » 
of  all  that  is  essential  in  the  book,  from  a  new  point  of  view. 
It  is  intended  to  do  for  the  whole  book  what  the  suggestive 
topics  at  the  end  of  each  chapter  may  do  for  the  chapters. 

6.  The  headings  of  paragraphs  are  printed  in  black-faced 
type,  in  order  to  emphasize  the  analysis  of  subject  matter. 
Technical  terms  are  in  italics  the  first  time  they  appear.     The 
author  does  not  agree  that  all  technical  language  should  be 
omitted  from  even  an  elementary  course. 

The  author  extends  most  cordial  thanks  to  the  many  pub- 
lishers and  authors  whose  courtesy  enables  him  to  reproduce 
classic  illustrations  from  their  copyrighted  works.  Recognition 
is  given  to  the  sources  in  immediate  connection  with  the  figures. 
The  thanks  of  the  author  are  also  due  to  many  fellow  teachers 
for  suggestions  and  criticisms  during  the  progress  of  the  work, 
and  since  the  appearance  of  the  second  edition. 

T.W.  GALLOWAY. 
JAMES  MILLIKIN  UNIVERSITY. 


CONTENTS 


CHAPTER  I 

PAGE 
INTRODUCTION I 

CHAPTER  II 
PROTOPLASM:  ITS  MORPHOLOGY  AND  PHYSIOLOGY 8 

CHAPTER  III 
THE  ANIMAL  CELL:  ITS  MORPHOLOGY  AND  PHYSIOLOGY 19 

CHAPTER  IV 
FROM  THE  SIMPLE  CELL  TO  THE  COMPLEX  ANIMAL 29 

CHAPTER  V 
CELLULAR  DIFFERENTIATION. — TISSUES 47 

CHAPTER  VI 
THE  GENERAL  ANIMAL  FUNCTIONS  AND  THEIR  APPROPRIATE  ORGANS    .    .     65 

CHAPTER  VII 
PROMORPHOLOGY 91 

CHAPTER  VIII 
DIFFERENTIATION  OF  INDIVIDUALS  AND  ADAPTATION 99 

CHAPTER  IX 
A  GENERAL  REVIEW  OF  THE  ANIMAL  KINGDOM    . 142 

CHAPTER  X 
PHYLUM  I. — PROTOZOA  (Primitive  Animals)      149 

CHAPTER  XI 

PHYLUM  II. — PORIFERA  (Pore  Bearing) 164 

xi 


Xll  CONTENTS 

CHAPTER  XII 

PAGB 

PHYLUM  III. — CCELENTERATA   (HOLLOW  INSIDE)   (Hydroids,  Corals,  Jelly- 
fishes,  Etc.) 173 

CHAPTER  XIII 

UNSEGMENTED  WORMS  (at  least  four  Phyla,  including  Flat-worms,  Thread- 
worms, Rotifers,  Polyzoa,  Etc.) 190 

CHAPTER  XIV 

PHYLUM  VIII. — ECHINODERMATA  (Starfish,   Sea-urchins,   Sand-dollars,   Sea- 
lilies) 207 

CHAPTER  XV 
PHYLUM  IX. — ANNUL  ATA  (Segmented  Worms) 222 

CHAPTER  XVI 
PHYLUM  X. — MOLLUSCA 239 

CHAPTER  XVII 
PHYLUM  XI. — ARTHROPODA 265 

CHAPTER  XVIII 
PHYLUM  XII. — CHORDATA 311 

CHAPTER  XIX 

CHORDATA  (CONT.)  :  SUB-PHYLUM  VERTEBRATA  (Fishes,  Amphibians,  Reptiles, 

Birds,  and  Mammals) .•    •    •   3J5 

CHAPTER  XX 
CLASS  I.— PISCES       358 

CHAPTER  XXI 
CLASS  II. — AMPHIBIA  (Frogs,  Toads,  Salamanders) .   375 

CHAPTER  XXII 

CLASS  III.— REPTILIA  (Lizards,  Crocodiles,  Tortoises,  Snakes) 384 

CHAPTER  XXIII 
CLASS  IV.— AVES  (Birds)      398 

CHAPTER  XXIV 
CLASS  V. — MAMMALIA  (Mammals) 437 


CONTENTS  Xlll 

CHAPTER  XXV 

PAGE 

CLASS  MAMMALIA  (CONT.):  Man 465 

CHAPTER  XXVI 
THE  DOCTRINE  OF  EVOLUTION  AND  RELATED  IDEAS 476 

CHAPTER  XXVII 
ECONOMIC  ZOOLOGY 498 

CHAPTER  XXVIII 
DEVELOPMENT  OF  ZOOLOGY 508 

CHAPTER  XXIX 
EXERCISES  IN  COMPARATIVE  PHYSIOLOGY,  MORPHOLOGY,  AND  ECOLOGY  .    .    .  520 

APPENDIX 

LABORATORY  SUGGESTIONS 523 

INDEX 533 


A  TEXT-BOOK  OF  ZOOLOGY 


CHAPTER  I 

INTRODUCTION 

1.  Nature  presents  to  man,  as  he  looks  upon  it,  a  great  and 
interesting  variety  of  material  objects.     Each  member  of  the 
race  gathers  in  his  lifetime,  by  means  of  experience  and  infer- 
ence, a  certain  limited  knowledge  of  these  objects  and  of  the 
changes  which  they  undergo.     The  knowledge,  thus  collected 
and  systematized  in  the  course  of  the  history  of  the  human 
race,   constitutes  the  so-called  Natural  Sciences.     Every  one 
of  us,  whether  he  deliberately  chooses  or  not,  must  be  in  some 
degree  a  natural  scientist.     The  beauty  and  interest  of  the 
work  has  attracted  and  charmed  thousands  of  people  of  all 
conditions,  in  all  parts  of  the  world. 

We  commonly  speak  of  material  objects  as  either  living  or 
non-living — as  organic  and  inorganic.  The  study  of  living 
things  in  all  their  relations  we  call  Biology.  Physics  and  Chem- 
istry are  often  considered  as  dealing  exclusively  with  inor- 
ganic matter,  and  are  therefore  placed  in  contrast  with  Biology. 
Their  principles  apply,  however,  in  the  realm  of  living  things 
just  as  truly  as  in  the  non-living,  and  one  must  not  imagine 
because  of  this  antithesis  that  the  phenomena  of  life  can  be 
explained  apart  from  chemical  and  physical  laws.  The  term 
Biology  was  first  introduced  about  the  beginning  of  the  nine- 
teenth century,  and  is  intended  to  express  the  fact  that  plants 
and  animals  are  similar  in  their  most  essential  structure  and 
activities.  The  term  Natural  History  is  sometimes  used 
synonymous  with  Biology. 

2.  Zoology. — Owing    to    the    fundamental    likeness    of    all 
living  matter,  there  is  great  theoretical  difficulty  in  distinguish- 
ing between  the  plant  and  animal  kingdoms.     The  practical 


2  ZOOLOGY 

difficulty  however  is  confined  to  the  very  lowest  and  simplest 
forms  of  life.  The  plants  and  animals  which  come  under  the 
common  observation  of  the  student  are  readily  distinguished^ 
It  is  only  the  deeper  study  which  reveals  the  underlying  simi- 
larity of  all  living  objects.  The  branch  of  Biology  which 
treats  of  plants  is  called  Botany;  that  which  deals  with  animals, 
Zoology. 

3.  The  Value  of  the  Study  of  Zoology.— The  student,  on 
taking  up  a  new  subject  has  a  perfect  right  to  ask  whether  that 
subject  is  worth  while.  A  subject  may  have  a  great  deal  of 
value  in  practical  ways  and  not  mean  much  in  education;  or 
it  may  have  high  value  in  educating  human  beings  and  have 
very  little  of  practical  worth.  Zoology  is  strong  in  both  par- 
ticulars. Animals  constitute  one  of  the  most  interesting  and 
important  features 'in  the  surroundings  of  man.  They  arouse 
our  interest,  they  appeal  to  our  sense  of  beauty,  they  furnish 
us  with  food  and  clothing,  they  attack  our  crops,  they  produce 
diseases  in  us  and  in  those  animals  we  most  use.  In  the  second 
place,  Zoology  adds  to  our  knowledge  of  the  structure  and 
activities  of  man  himself  since  we  are  enabled  through  it  to 
study  ourselves  in  proper  relation  to  the  other  animals.  We 
may  gain  much  light  on  the  means  of  preserving  human  health 
and  making  right  adjustments  by  the  study  of  animals.  Finally, 
the  very  method  by  which  we  study  zoology  is  of  the  greatest 
value  in  our  mental  growth.  The  scientific  method  demands 
that  we  observe  at  first  hand  as  many  facts  as  possible;  that  we 
compare  and  contrast  these  facts  with  one  another  and  with 
those  which  other  people  have  observed;  that  we  discriminate 
between  important  and  unimportant  facts;  that  we  learn  to 
draw  right  conclusions  from  our  facts ;  and  that  we  always  hold 
our  minds  open  for  new  facts  even  after  we  have  reached  our 
conclusions.  It  is  worth  while  to  get  these  ideals  and  form 
these  habits. 

To  the  investigator,  the  ultimate  object  of  zoological  study 
is  to  find  the  real  nature  of  animal  life  as  it  exists,  the  mode  of 
its  development,  and  the  causes  which  have  brought  it  to  its 
present  exquisite  variety  and  adjustment.  These  larger  and 


INTRODUCTION  3 

more  general   questions  constitute   what  may  be   called   theo- 
retical Zoology,  or  the  principles  of  Zoology. 

4.  Practical  Exercises. — Cause  the  student  to  select  ten  or  more  kinds  of 
animals  with  which  he  is  partially  acquainted,  and,  from  his  observation  and  ex- 
perience, to  enumerate  the  points  at  which  they  touch  human  welfare.  Are  they, 
in  each  instance,  to  be  classed  as  helpful?  as  harmful?  or  merely  as  indifferent? 
Is  their  influence  upon  man's  interest  direct  or  indirect? 

What  animals  have,  in  the  past,  most  appealed  to  your  interest?  Select  that 
particular  quality  in  which  you  have  been  most  interested  (structure,  beauty, 
powers,  instincts,  habits)  and  show  how  the  attempt  to  study  or  explain  any  one 
takes  you  at  once  into  all  the  others. 

5.  Divisions    of    the    Science. — The    facts    and    principles 
which  have  been,  and  are  yet  to  be,   discovered  concerning 
animals  are  so  numerous  and  various  in  their  bearings,  and  in- 
vestigators approach  the  subject  from  such  different  points  of 
view  that  it  is  necessary,  in  order  to  express  these  results,  to 
divide  zoology  into  several  branches  or  departments.     It  must 
be  held  in  mind,  however,  that  these  divisions  are  more  or  less 
artificial,  and  that  the  facts  of  each  department  are  to  be  con- 
sidered in  connection  with  those  of  all  the  others,  if  they  are 
really  to  be  understood.     With  all  its  departments,  animal  life 
is  to  be  thought  of  as  a  whole.     Structures  exist  for  the  per- 
formance of  function,  and  the  activities  are  intended  to  adjust 
the  animal  to  its  whole  life-relation. 

6.  Morphology  is  the  branch  of  the  science  which  deals 
with  form  or  structure  in  its  broadest  sense,  whether  internal 
or  external,  partial  or  total.     In  its  most  general  sense  it  em- 
braces the  study  of  animals  from  the  standpoint  of  symmetry, 
— that  is,  the  form  of  the  organism  with  reference  to  certain 
planes  passing  through  the  body.     For  example,  the  human 
body  may  be  so  divided  by  a  single  plane  that  two  essentially 
similar  parts  result, — the  right  and  the  left.     Again  similar 
parts  may  succeed  each  other  in  a  linear  series,  as  in  the  seg- 
ments of  the  earth-worm;  or  they  may  radiate  from  a  central 
point,  as  in  the  arms  of  the  star-fish.     This  is  the  most  funda- 
mental kind  of  morphology.     It -relates  the  organism  to  space. 
It  is  called  Promorphology,  and  is  related  to  Zoology  some- 
what as  the  study  of  crystals  is  to  Mineralogy. 

Anatomy  is  that  department  of  morphology  which  treats 


4  ZOOLOGY 

of  the  structure  of  parts  of  the  individual, — as  the  organs  and 
systems  of  organs,  the  tissues,  the  cells,  and  so  forth.  This  is 
known  as  gross  anatomy  if  the  study  pertains  to  the  larger 
units, — as  organs;  it  is  called  Histology,  if  the  constituent  ele- 
ments of  these  organs  (as  tissues  and  cells)  are  to  be  considered. 
Thus  far  we  have  thought  of  structure  as  stationary  or  per- 
manent. As  a  matter  of  fact  we  know  that  each  organism 
begins  life  in  a  very  modest  way,  as  a  single  "cell,"  and  grows 
more  complex  by  fairly  well-defined  stages  until  the  adult  con- 
dition is  reached.  This  is  development.  The  science  of 
Embryology  is  the  record  of  this  history  of  the  successive  stages 
which  the  individual  animal  assumes  in  becoming  adult,  or 
at  least  until  its  organs  are  essentially  formed. 

7.  In  Physiology  are  considered  the  facts  and  laws  relating 
to  the  activities  or  functions  of  the^  organism  and  of  its  separate 
parts.     It  includes  the  tracing  back  of  the  adult  activities  to 
their  lowest  form,   as  found  in  the  simplest  animals  or  the 
youngest  stages  of  the  higher  animals.     It  includes  the  powers 
of  the  single  cell;  the  chemical  and  physical  processes  which 
seem  to  underlie  all  the  functional  activities;  the  division  and 
more  perfect  performance  of  the  primitive  functions  as  the 
various    organs    arise    and    come    to    do    their    special    work. 
Finally  it  includes  the  relation  of  the  animal  as  a  whole  to  other 
animals  of  the  same  or  of  different  species,  to  plants,  and  to 
the  inanimate  surroundings.     The  term  Ecology  is  applied  to 
the  branch  of  physiology  which  treats  of  the  relation  of  the 
organism  to  the  complex  and  wonderful  conditions  in  which 
it  finds  itself.     Of  recent  years  much  emphasis  is  being  given 
to  this  branch  of  zoology. 

8.  Animals  may  be  studied  as  to  their  distribution  or  occur- 
rence in  the  world.     For  example,  we  find  lions  in  Africa  and 
Asia  only,  and  the  African  and  Asiatic  lions  are  of  different 
varieties;  the  giraffe  is  found  only  in  Africa;  man  is  found 
over  the  most  of  the  habitable  globe,  but  before  the  era  of  easy 
communication  between  distant  countries  the  men  of  different 
regions  were  conspicuously  different.     Again  we  can  easily  see 
that  the  animals  that  live  in  the  various  bodies  of  water  are 


INTRODUCTION  5 

very  different  from  those  living  on  the  land;  those  in  the  frigid 
zones  are  different  from  those  in  the  temperate  and  torrid. 
All  such  topics  are  treated  under  the  head  of  distribution,  or 
geographical  distribution.  This  is  distribution  in  space. 

The  hard  parts  of  animals  are  found  as  fossils  in  many  of 
the  surface  rocks  of  the  earth.  The  various  systems  of  rock- 
strata  are  characterized  by  more  or  less  different  fossil  remains, 
indicating  a  variation  in  the  animal  life  during  the  successive 
periods  of  the  earth's  history.  This  distribution  of  animals 
in  time  is  the  subject-matter  of  Palao  zoology.  The  facts 
of  palaeozoology  and  the  conclusions  resting  thereon  are 
among  the  most  important  in  the  whole  realm  of  Zoology, 
inasmuch  as  they  supplement  the  facts  gained  from  the  study 
of  embryology  and  morphology  of  living  species,  thus  enabling 
the  investigator  to  trace  the  history  of  the  various  races  of 
animals  into  the  remote  past.  In  this  way  we  also  learn  much 
of  the  history  of  the  earth  itself. 

9.  Practical  Exercises. — Let  the  student  submit  a  written  report  on  the  dis- 
tribution of  the  animals  in  his  immediate  neighborhood,  based  on  his  own  observa- 
tions. The  report  need  not  be  exhaustive  in  order  to  convince  the  student  of  the 
effect  of  the  environment,  which  includes  everything  in  the  surroundings,  on  the 
distribution  of  animals.  Some  classification  should  be  made  of  the  varieties  of 
territory  included; — as  river,  pond,  lowland,  woodland,  prairie,  mountain,  and  the 
like.  Determine,  by  reference  to  the  authorities  available,  the  geographical 
distribution  of  the  following:  the  elephant,  the  camel,  the  kangaroo,  the  horse, 
the  white  bear,  the  seal,  the  salmon,  the  crocodile,  the  reef-forming  coral,  the 
sponge  of  commerce.  Select  five  other  species  having  a  personal  interest. 

10.  Classification. — In  studying  animals  and  plants  one  is 
soon  impressed  with  the  fact  that  among  the  thousands  of 
individuals,  even  of  the  same  general  kind,  there  are  no  two 
exactly  alike;  and  yet  among  them  all,  with  their  manifest  dif- 
ferences, there  are  numerous  points  of  similarity.  These  two 
facts  make  it  possible  to  group  those  most  alike  into  more  or 
less  coherent  classes,  separating  them  at  the  same  time  from 
other  classes.  The  forming,  naming,  and  defining  of  these 
groups  and  subgroups  we  call  Taxonomy  or  Classification. 
Manifestly,  true  classification  must  depend  upon  the  facts  de- 
rived from  the  completest  possible  study  of  the  structure  and 
relations  of  organisms,  and  can  only  be  perfect  when  we  know 


6  ZOOLOGY 

all  that  is  to  be  known  about  them.  In  addition  to  displaying 
our  present  knowledge  of  the  relationship  of  animals,  classifi- 
cation serves  a  most  important  end  in  giving  us  more  rapid 
power  of  using  that  knowledge  in  getting  further  knowledge 
that  is  needed. 

11.  Historical. — Zoology  as  a  science  can  scarcely  be  said 
to  be  more  than  three  hundred  years  old,  although  Aristotle, 
more  than  three  hundred  years  before  Christ,  wrote  much  of 
value  concerning  animals.     Later  many  facts  of  general  anat- 
omy were  discovered  in  connection  with  the  study  of  medicine, 
and  about  1600  the  invention  of  the  microscope  opened  up  the 
field  of  histology.     Toward  the  end  of  the  seventeenth  century 
an  effort  was  made  to  establish  a  scientific  classification  of 
animals.     Since  that  time  very  much  of  the  attention  of  students 
of   zoology  has  been  turned  in   this   direction.     During  the 
last  century  however  there  has  been  a  constantly  increasing 
interest  in  the  study  of  embryology,  of  histology,  and  in  the 
general  theoretical  questions,   the  answers  to  which  depend 
on  the  bringing  together  of  the  results  of  studies  in  all  depart- 
ments.    Such  are  the  problems  of  race  development  or  evolu- 
tion, of  heredity,  of  man's  place  in  nature,  and  the  like.     The 
most  notable  development  of  the  subject  in  recent  years  has 
been  in  connection  with  the  study  of  the  finer  structure  of  the 
cell,  in  more  exact  methods  of  studying  physiology,  and  in 
extending  its  scope  to  take  in  the  lower  organisms  as  well  as 
the  higher  and  the  single  cell  as  well  as  the  organs.     It  is 
important  to  add  that  all  this  work  is  now  being  done  in  a 
comparative  way.     The  necessity  of  comparing  the  histology, 
the  embryology,  and  the  physiology  of  one  animal  with  that 
of  another  arises  from  the  belief  in  the  unity  of  animal  life, 
and  that  all  animals  are  really  akin.     If  animals  of  different 
kinds  are  really  related,  their  likenesses  and  differences  take  on 
a  new  meaning  to  the  student,  and  classification  comes  to  ex- 
press the  degree  of  kinship,  as  well  as  to  serve  the  convenience 
of  the  investigator. 

12.  Summary. 

I.  Natural  Science  embraces: 


INTRODUCTION  7 

A.  The  sciences  of  inanimate  things — 

Astronomy, 
Geography, 

Meteorology,  Mineralogy,  Lithology,  etc. 

B.  The  sciences  of  animate  things — 

Botany, 
Zoology. 

(Physics  and  Chemistry  are  fundamental  to  both 
groups  of  sciences;  Geology  embraces  portions  of 
the  subject-matter  of  both  groups.) 

II.  Subdivisions  of  Zoology. 

A.  Morphology: 

1.  Promorphology,  which  treats  of  general  form; 

2.  Anatomy;  =  the  structure  of  parts; 

Gross  =  structure  of  organs  and  systems  of 
organs; 

Microscopic  =  (Histology,  Cytology);  struc- 
ture of  tissues  and  cells; 

3.  History  of   Development    (structural  stages): 

Individual  =  (Embryology,  Ontogeny); 
Racial  =  (Phylogeny). 

B.  Physiology: 

1.  Physiology  proper;  =  the  functional  relation  of 
part  to  part  and  to  the  whole; 

2.  Ecology;  =  relations   of   the  individual  to  its 
whole  surroundings. 

C.  Distribution: 

1.  In  space  =  (Geographical  Distribution); 

2.  In  time  =  (Palaeozoology,  as  revealed  by  fossils) . 

D.  Classification,  or  the  grouping  of  animals  accord- 
ing to  their  likeness  or  kinship. 

E.  Economic  zoology  considers  all  the  points  at  which 
animals  touch  human  welfare. 


CHAPTER  II 

PROTOPLASM:  ITS  MORPHOLOGY  AND  PHYSIOLOGY 

13.  Life. — Life  may  be  thought  of  in  two  somewhat  distinct 
ways.     It  may  be  considered,  first,  merely  as  an  expression 
for   all   the   various   activities  of  the   organism, — the   sum  of 
all  the  phenomena  of  its  existence;  or,  second,  as  a  force  or 
form  of  energy  from  which  the  special  modes  of  activity,  as 
feeding,   growth,   motion,   and  thinking,   arise.     The  latter  is 
the  more  common  use  of  the  term,  and  yet  the  former  is  the 
only  use  of  it  which  can  be  completely  justified.     Much  of  the 
activity   of   living  things  may  be  explained  by  reference  to 
the  ordinary  physical  and  chemical  laws.     What  we  mean  by 
this  is  that  all  the  so-called  vital  activities  depend  on  such 
physical  facts  as  cohesion  and  adhesion,  on  the  laws  of  fluids 
and  of  solution  of  solids  in  fluids;  and  on  such  chemical  facts 
as  the  building  up  and  tearing  down  of  chemical  compounds 
composed  of  just>  the  same  elements  that  we  find  about   us 
everywhere  in  the  world.     There  are  a  great  many  biologists 
who  think  that  there  is  nothing  more  in  life  than  these  physical 
and  chemical  processes  and  effects,  and  that  we  should  be  able 
to  explain  life  and  its  activities  if  we  knew  all  about  the  physics 
and  chemistry  of  living  matter.     It  is  sure,  however,  that  there 
are  many  vital  phenomena   which   cannot   be   even   remotely 
explained  by  what  we  now  know  of  chemistry.     Probably  this 
will  always  be  so. 

14.  Living  and  Non-living  Objects.— Any  one  of  us  could, 
almost  at  a  glance,  tell  whether  an  object  is,  or  has  been,  alive; 
and  yet  it  is  not  easy  to  describe  just  what  it  is  that  convinces 
us.     There  are  two  classes  of  differences  between  living  things 
and  things  that  have  never  been  associated  with  life;  differences 
(i)  in  make-up  or  organization,  and  (2)  in  powers. 

8 


PROTOPLASM  9 

Organization  includes  form,  size,  organs  or  parts,  as  well  as 
minute  structure.  While  no  two  organisms  are  of  exactly  the 
same  form,  there  are  certain  features  that  are  very  common. 
Living  bodies  are  usually  bounded  by  curved  surfaces.  They 
tend  to  be  elongated  in  one  axis  more  than  in  others;  they  tend 
to  repeat  certain  of  their  parts;  and  they  incline  to  be  sym- 
metrical. They  are  limited  in  size, — although  some  are  very 
small  and  some  are  very  large.  They  are  differentiated, — that 
is,  they  have  organs,  different  kinds  of  structures  at  different 
parts  of  the  body.  As  we  shall  see  later  they  are  made  up  of  a 
substance  called  protoplasm,  which  is  arranged  in  one  or  more 
cells. 

Among  the  powers  and  activities  of  a  living  object  may  be 
mentioned  (i)  the  power  of  changing  food  into  its  own  substance, 
and  thus  of  growth  and  repair;  (2)  the  power  of  using  this 
growth  to  separate  off  a  portion  and  thus  make  a  new  individual 
like  itself  (reproduction);  (3)  the  power  to  use  some  of  the 
material  of  growth  to  develop  energy  of  motion,  heat,  light, 
electricity,  or  thought,  as  the  case  may  be;  and  (4)  the  ability 
by  means  of  sensitiveness  and  the  use  of  these  various  powers 
to  adjust  itself  to  very  considerable  changes  in  the  environment. 
Every  organism  has  all  these  qualities  in  some  measure.  No 
inorganic  object  has  them. 

15.  The  Relation  of  Protoplasm  to  Life. — Whatever  life 
may  be,  in  the  last  analysis,  we  never  observe  its  manifestations 
except  in  connection  with  a  substance  called  protoplasm,  which 
is  found  both  in  plants  and  animals.  Protoplasm  does  not  con- 
tain any  chemical  elements  which  are  not  found  in  other  than 
living  materials.  Notwithstanding  this  fact,  protoplasm  is 
different  from  any  other  known  substance.  It  is  more  com- 
plex and  more  highly  organized,  as  to  its  machinery,  than  any 
other  chemical  or  physical  compound  whatsoever.  Protoplasm 
has  the  power  of  growing  by  taking  up  and  changing  other 
non-living  substances;  but,  so  far  as  we  know,  it  is  never  pro- 
duced except  as  the  result  of  the  growth  and  division  of 
antecedent  protoplasm.  The  protoplasmic  or  living  material 
in  an  organism  is  normally  composed  of  a  number  of  unit- 


10  ZOOLOGY 

masses  called  cells  (see  Chapter  III).  These  unit-masses  of 
protoplasm  are  in  some  degree  independent  of  one  another, 
because  normally  each  tends  to  form  a  wall  about  itself;  and 
yet  it  is  highly  probable  that  the  whole  protoplasm  of  an  ani- 
mal is  physically  continuous  by  means  of  delicate  connections 
between  the  units.  The  life  of  the  cells  is  not  quite  the  same 
thing  as  the  life  of  the  organism  to  which  they  belong,  for 
in  animals  composed  of  more  than  one  cell  a  cell  may  die  with- 
out involving  the  death  of  the  animal.  The  protoplasm  of  the 
cell  may  also  retain  life  for  a  time  after  separation  from  the 
living  animal  or  after  the  animal  as  a  whole  has  ceased  to  live. 
This  is  shown  by  the  fact  that  cells  may  be  taken  from  the 
body  of  young  organisms  and,  if  kept  nourished  under  conditions 
similar  to  those  of  the  body  from  which  they  are  taken,  will  not 
merely  live  but  will  continue  to  divide  and  grow  almost 
indefinitely.  This  has  been  shown  to  be  true  of  several  classes 
of  cells. 

1 6.  Protoplasm. — While  we  describe  protoplasm  as  the 
"physical  basis  of  life"  (Huxley),  we  no  longer  think  of  it  as  a 
constant  or  definite  material  with  an  exact  composition.  It 
is  rather  a  complex  mixture  of  substances,  some  of  which  are 
themselves  very  complex  compounds.  These  substances  in 
the  mixture  are  continually  bringing  about  changes  in  one 
another.  It  is  inevitable  that  such  a  mixture  of  changeable 
substances  should  itself  be  most  unstable  and  variable.  It  is 
agreed  that  there  is  much  in  common  in  all  protoplasm, — even 
in  protoplasm  as  far-  apart  as  that  of  plants  and  animals;  and 
yet  it  is  also  true  that  the  protoplasms  of  different  animals  and 
of  different  parts  of  the  same  animal  are  definitely  different. 
This  difference  is  ^hown  by  the  difference  in  the  work  they  can 
perform, — as  in  muscles  and  nerve  cells. 

Perhaps  this  is  the  most  wonderful  thing  in  life :  that  a  sub- 
stance, so  complex  and  so  variable  that  it  is  not  exactly  the 
same  any  two  moments  in  succession,  should  still  be  so  constant 
that  a  small  amount  of  it  split  off  generation  after  generation 
should  transmit  all  the  essential  qualities  of  the  species  to  which 
it  belongs, — whether  a  paramecium  or  a  man  or  an  oak. 


PROTOPLASM  1 1 

17.  Chemical  Composition  of  Protoplasm. — It  is  impossible  to  make  a  satis- 
factory chemical  analysis  of  protoplasm,  as  it  loses  its  characteristic  powers  and 
probably  undergoes  important  chemical  and  physical  changes  in  the  act  of  analysis. 
The  dead  material  thus  obtained  is  no  longer  the  substance  with  which  we  started, 
either  as  to  its  power  or  its  structure.     The  experiment  shows,  however,  that  the 
substance  is  both  chemically  and  physically  unstable.     By  an  analysis  of  the  dead 
protoplasm,  we  find  present  several  complex  organic  compounds,  known  as  proteids, 
carbohydrates  (starches  and  sugars),  fats,  ferments,  pigments,  etc.     In  addition  to 
these  are  simpler  inorganic  compounds,  as  water  and  various  salts.     Doubtless 
some  of  these  materials  are  food-substances  on  their  way  to  form  protoplasm,  and 
others  are  the  waste-products  of  protoplasmic  disruption,  ready  to  be  cast  out  of 
the  cell.     The  proteids  are  the  most  complex  of  all  these  substances  and  it  is 
believed  that  protoplasm  finds  its  real  basis  in  these. 

The  proteids  are  various  in  composition  and  properties,  but  agree  in  that  their 
molecules  contain  carbon,  hydrogen,  oxygen,  nitrogen,  and  sulphur,  in  proportion 
roughly  as  follows:  C  53%,  O  22%,  N  17%,  H  7%,  S  i%.  Qarboji  is  thus  the 
most  important  single  constituent  element.  The  white  of  egg,  the  fibrin  of  the 
blood,  and  casein  in  milk  are  examples  of  proteid. 

Carbohydrates  consist  of  C,  H,  and  O.  The  latter  elements  are  always  present 
in  the  ratio  in  which  they  are  represented  in  water  (H20),  'e.g.  CeHioOs.  The 
starches,  sugars,  and  cellulose,  such  as  is  found  in  cotton  fibres,  are  illustrations. 

The  fats  contain  the  same  elements  as  starch,  but  the  percentage  of  oxygen 
in  terms  of  the  hydrogen  is  much  smaller  than  in  the  starches. 

The  ferments  are  complex  organic  substances  which  have  the  power  of  produc- 
ing important  chemical  changes  in  other  substances  without  being  themselves 
consumed.  They  play  an  important,  but  not  thoroughly  understood,  r61e  in  the 
activities  of  the  organisms,  both  within  and  outside  the  cells  which  produce  them. 
The  active  principle  of  the  digestive  juices,  as  ptyalin  and  pepsin,  are  examples  of 
ferments  which  have  been  extruded  from  the  cells. 

Water  (H2O)  is  very  important  in  both  the  chemical  and  physical  structure  of 
protoplasm.  It  is  very  variable  in  amount,  and  the  degree  of  activity  of  the 
protoplasm  is  roughly  proportional  to  the  amount  of  water  present.  Traces  of 
inorganic  salts, — compounds  of  chlorine,  potassium,  sodium,  calcium,  phosphorus, 
iron,  etc.,  are  also  found  in  solution  in  the  water. 

Most  of  these  substances  cannot  be  considered  as  "living."  The  water  and 
inorganic  salts  and  starch  cannot  be.  The  starches  and  fats  and  urea  are  organic 
but  not  living,  If  any  particular  substances  are  alive  it  would  seem  to  be  the 
proteins.  It  may  be,  however,  that  life  is  the  result  of  the  intimate  relations  and 
interactions  of  all  these  various  substances  rather  than  a  property  of  any  one  of 
them. 

1 8.  The   Physical   Structure   of  Protoplasm. — This   varies 
much  from  time  to  time.     On  account  of  differences  in  the 
amount  of  water  present,  the  consistency  of  protoplasm  may 
vary  from  the  quite  fluid  condition  found  in  actively  growing 
parts,  to  the  very  much  more  solid  condition  apparent  in  dry 
seeds  and  in  the  resting  or  encysted  stage  of  some  animals. 


12  ZOOLOGY 

In  these  latter  instances  the  protoplasm  eliminates  a  large  per 
cent,  of  its  water,  forms  a  thick  wall,  and  thereby  becomes 
enabled  to  resist  drouth  and  heat  and  cold  as  it  could  not  possibly 
do  otherwise.  Under  ordinary  circumstances  protoplasm  ap- 
pears as  a  semi-fluid  or  gelatinous  material. 

Concerning  the  architecture  of  protoplasm  there  is  much 
diversity  of  opinion.  It  seems  probable  that  this,  like  the 
chemical  composition,  is  subject  to  considerable  variation.  It 
is  certainly  very  complicated  and  represents  several  physic- 
ally distinct  substances  mingled  in  a  very  effectual  and 
wonderful  way.  In  some  cases  at  least  these  take  on  the  ap- 
pearance of  a  foam  structure  such  as  is  obtained  in  an  emul- 
sion of  oil  in  water,  or  of  air  and  water  in  a  soapy  lather. 
Whatever  its  form  may  be,  it  seems  that  there  must  be  a  close 
relation  between  the  architecture  and  the  great  activity  which 
protoplasm  shows.  It  is  physically  as  well  as  chemically 
unstable. 

19.  Physiology   of  Protoplasm. — The  mass   of   protoplasm 
which  we  have  called  a  cell,  or  unit,  performs  practically  all 
the  functions  shown  by  the  more  complex  organism.     It  has 
the  power  of  feeding,  of  growth,  of  reproduction,  of  motion  in 
response  to  stimuli,  and  of  waste  and  repair.     Even  in  the  higher 
animals,  made  up  of  many  of  these  units,  the  processes  are 
performed,   on  last  analysis,   by  the  individual  protoplasmic 
units  of  which  the  body  is  composed. 

20.  Irritability. — Owing   to   its   chemical   and   physical   in- 
stability,   living    protoplasm    is    constantly    changing.     These 
changes  may  be  the  direct  result  of  internal  or  external  con- 
ditions to  whose  influence  the  protoplasm  may  respond  by  a 
manifestation   of   energy   greater   than   that   involved'  in   the 
stimulus.     This  quality  is  called  irritability.     It  further  seems 
that    changes    may    originate    within    the    protoplasm    itself, 
though  this  is  much  more  difficult  to  demonstrate  and  may 
merely  represent  our  ignorance  of  the  processes  occurring  in 
the  protoplasm.     This  power  is  called  automatism.     These  are 
the  most  fundamental  qualities  belonging  to  protoplasm,  and 
serve  to  make  possible  those  which  follow:  viz.,  motion,  assimi- 


PROTOPLASM  13 

lation,  growth,  etc.  Protoplasm  varies  in  the  degree  of  irri- 
tability. In  general  it  responds  to  stimuli  most  normally  under 
those  conditions  which  are  most  favorable  to  the  ordinary  vital 
processes. 

21.  Stimuli. — All  the  disturbing  forces  or  conditions,   ex- 
ternal or  internal,  which  tend  to  cause  response  in  living  pro- 
toplasm,   are    called    stimuli.     The    principal    stimuli    are, — 
chemically   active  substances,   moisture,  contacts,  heat,  light, 
electricity,    and    gravity.      Inasmuch    as    irritability    lies    at 
the   foundation    of    the   various  protoplasmic  activities  men- 
tioned below,  all  the  natural  causes  which  modify  irritability, 
also  modify,  through  it,  the  vital  processes,  such  as  motion, 
growth,  etc. 

Light  affects  protoplasm  profoundly.  The  direction  of  motion  in  protoplasm 
is  largely  determined  by  light.  Light  may  either  attract  or  repel  protoplasm. 
Excess  of  light  retards  growth.  Heat  strongly  modifies  the  rate  of  all  the  vital 
processes.  There  is  an  optimum  temperature  at  which  the  protoplasm  best 
performs  its  work.  An  excessive  increase  or  decrease  of  this  temperature  produces 
a  cessation  of  activity,  a  condition  of  rigor,  and  death.  The  fatal  maximum 
temperature  for  ordinary  animal  protoplasm  may  be  said  to  be  about  45°  or  50°  C. ; 
the  minimum,  o°,  or  below.  ("Chemical  agents  may  stimulate  protoplasm  in  such  a 
way  as  to  attract  or  repel  organisms.  Paramecia,  which  are  single-celled  animals, 
may  be  seen  to  gather  about  an  air-bubble,  or  at  the  margin  of  the  cover-glass. 
They  will  retreat  before  an  encroaching  solution  of  certain  salts. 

It  is  a  most  significant  fact  in  this  connection  that  protoplasm  may  become,  so 
to  speak,  accustomed  to  a  stimulus  which  has  been  long  continued,  so  that  it  ceases 
to  respond  in  the  way  it  did  when  the  stimulus  was  novel.  Protoplasm  may 
gradually  be  brought,  for  example,  to  endure  and  thrive  at  a  temperature  which 
would  have  produced  death  if  suddenly  applied.  It  is  almost  impossible  to  over- 
state the  importance  of  this  faculty  in  enabling  organisms  to  survive  changing 
conditions.  Stimuli,  then,  may  be  said  to  be  powerful  in  proportion  to  their 
suddenness  and  intensity. 

22.  Assimilation. — The  process  of  changing  food  substances 
into   protoplasm   is    called    assimilation.     It    can   be   effected 
only   by   protoplasm.     Such   foods   may   be   relatively   simple 
substances    or   may    be    the    complex    protoplasm    of    other 
organisms.     The  protoplasm  of  the  green  leaves  of  plants  has 
the  power   of   utilizing   the   simple  inorganic   compounds,    as 
oxygen,  water,  and   carbon   dioxid,  in  a  larger  measure  than 
that  of  animals.     Plants  may  build  these  up  into  foods,  whereas 
animals  must  have  the  complex  organised  foods  to  start  with. 


ZOOLOGY 


23.  Growth  and  Reproduction. — The  result  of  assimila- 
tion is  the  addition  of  new  molecules  of  complex  organic 
matter  among  the  molecules  of  the  old.  This  produces  growth. 
It  is  to  be  defined  as  increase  in  mass.  If  this  continues  in- 
definitely in  excess  of  whatever  may  tend  to  destroy  the  pro- 
toplasm, the  increase  in  size  may  lead  to  the  division  of  the 

FIG.  i. 

ANIMAL  SUBSTANCES  - 


REPRODUCTION 


OXIDATION 


SUN 


I.  ENERGY 
HEAT  . 
MOTION 
THOUGHT 


WAS! 


\  \ 

NITRITES        NITRATES 
CARBON  DIOXID 
^-O    WATER 

FIG.  i.  A  diagram  indicating  the  general  cycle  of  events  in  the  building  up  of  inorganic  sub- 
stances into  living  matter.  Sunlight  enables  the  chlorophyll  of  plants  to  build  up  water  and  carbon- 
dioxide  (photosynthesis)  into  carbohydrates.  Plants  may  use  these  together  with  nitrogenous  com- 
pounds to  form  plant  protoplasm  and  other  proteids.  Animals  may  by  assimilation  utilize  these  for 
growth,  reproduction,  and  in  doing  work  of  various  kinds.  Work  results  in  waste.  Plants  may 
again  use  these  wastes. 

Questions  on  the  figure. — What  are  the  three  main  courses  open  to  plant  sub- 
stances? In  what  sense  are  animals  dependent  on  plants?  For  what?  How 
important  is  photosynthesis  in  life?  What  is  the  real  source  of  the  energy  of 
organisms?  In  what  sense  is  the  term  cycle  appropriate  here? 

protoplasm.  The  parts  may  separate  and  lead  an  independent 
existence.  Such  is  reproduction.  In  its  simplest  form  it  is 
merely  growth  beyond  the  limits  of  the  individual.  The  cell 
cannot  continue  to  grow  indefinitely.  Its  size  is  limited  by  the 
necessity  of  physical  support  on  the  part  of  the  soft  protoplasm, 
and  by  the  relation  between  the  outer  surface,  through  which 


PROTOPLASM  15 

the  food  must  be  taken,  and  the  volume,  which  represents  the 
mass  to  be  fed.  The  surface  increases  as  the  square  of  the 
diameter,  whereas  the  volume  increases  as  the  cube  of  the 
diameter.  It  is  apparent  that  the  nourishing  surface  does 
not  increase  as  rapidly  as  the  mass  to  be  nourished,  and  in  con- 
sequence the  time  will  come  when  the  nourishment  possible  to 
be  absorbed  will  just  nourish  the  volume,  and  growth  must 
cease.  This  condition  may  constitute  an  internal  stimulus  to 
division.  At  any  rate  division  furnishes  a  way  out  of  the 
dilemma  and  allows  a  renewal  of  growth  of  the  daughter  units. 


P 

FIG.  2.  Streaming  of  Protoplasm  in  the  .Amoeba.  The  forward  motion  of  the  granules  takes 
place  more  rapidly  in  the  centre  of  the  pseudopodium  (p) .  Those  at  the  margin  fall  behind  those 
in  the  centre  as  the  pseudopodium  advances. 

Questions  on  the  figure. — Why  may  the  amoeba  readily  change  its  form?  Do 
its  internal  parts  preserve  a  constant  relation  to  each  other? 

24.  Contractility. — A  body  of  living  protoplasm  seems 
always  to  possess  the  ability  to  change  its  form  in  greater  or 
less  degree.  This  results  in  motion  of  parts  or  of  the  whole, 
and  is  called  contractility.  Movement  or  contractility  is  closely 
related  to  irritability,  and  results  from  the  action  of  stimuli, 
external  and  internal,  upon  the  complex  protoplasm.  It  is 
made  possible  by  the  assimilation  of  food  substances.  These, 
in  being  broken  down,  furnish  the  energy  shown  in  motion. 
The  nature  of  the  motion  resulting  from  contraction  differs 
somewhat,  depending  upon  whether  the  protoplasm  is  en- 
veloped by  a  cell-wall  or  is  naked.  If  without  a  wall,  it  may 
send  out  foot-like  projections  into  which  there  passes  a  stream 
of  granules,  as  in  the  Amoeba  (see  Fig.  2);  if  enclosed,  the 


i6 


ZOOLOGY 


protoplasmic  mass  may  rotate  within  the  cell  wall,  or  there 
may  be  narrow  channels  in  which  the  currents  move  between 
banks  of  more  stationary  material.  The  latter  motion  is  de- 
scribed as  circulation.  (Fig.  3.) 


FIG 


FIG.  3.  The  circulation  of  protoplasm  (p)  in  a  cell  of  a  stamen-hair  of  Tradescanlia.  In  the 
channels  the  granules  move  back  and  forth  to  the  various  parts  of  the  cell.  The  remainder  of  the 
cell  is  filled  with  cell-sap  ($)  which  in  these  cells  is  colored. 

Questions  on  the  figure. — In  what  respects  are  the  activities  of  the  protoplasm 
necessarily  limited  in  this  cell  as  compared  with  the  condition  in  Amoeba;?  Why  is 
circulation  an  appropriate  term? 

25.  Demonstrations. — The  teacher  should,  if  possible,  demonstrate  proto- 
plasmic motion  to  the  students  with  a  compound  microscope  of  good  magnification. 
The  Amoeba  will  serve  to  illustrate  the  naked  streaming  motion;  Paramecium, 
rotation;  the  hairs  from  the  stamens  of  Tradescantia  beautifully  illustrate  circula- 
tion. (There  is  a  cultivated  species  which  may  be  kept  blooming  in  greenhouses  at 
all  seasons  of  the  year.)  Ciliary  motion  may  be  shown  in  several  of  the  large 
Protozoa,  or  by  living  cells  scraped  from  the  esophagus  of  the  frog. 

26.  Dissimilation. — Motion  and  the  other  responses  which 
protoplasm  makes  to  stimuli  necessarily  represent  chemical  or 
physical  changes,  or  both,  in  the  protoplasm.  It  is  well  known 
that  complex  chemical  substances,  such  as  are  found  in  proto- 


PROTOPLASM  17 

plasm,  can  be  made  to  yield  energy  when  they  are  torn  down 
into  simpler  ones  by  some  element  which  has  an  affinity  for 
some  of  the  elements  constituting  the  substance.  The  result 
of  this  action  is,  always,  simpler  and  more  stable  compounds 
than  the  original,  and  therefore  of  less  use  in  the  further 
freeing  of  energy.  This  tearing-down  process  is  the  opposite 
of  assimilation  and  is  sometimes  called  dissimilation  or  katab- 
olism.  Oxygen  is  one  of  the  most  important  agents  in  nature 
for  the  freeing  of  energy  by  breaking  down  the  complex  chemical 
substances.  It  unites  with  the  carbon  and  hydrogen  particu- 
larly, and  these  unions  are  among  the  principal  sources  of 
energy  which  animals  show.  The  process  is  called  oxidation 
and  is  essentially  the  same  thing  that  occurs  when  wood  or  coal 
is  burned.  The  energy  belonging  to  the  wood  by  virtue  of  its 
chemical  constitution  is  partly  freed  by  the  action  of  the  oxygen 
in  uniting  with  the  carbon  and  hydrogen,  reducing  the  wood  to 
ashes,  water,  and  carbon  dioxid.  In  the  stove  the  principal 
form  of  energy  secured  is  heat;  but  in  appropriate  engines, 
locomotion  and  other  forms  of  mechanical  work,  or  light,  or 
electrical  energy  may  be  secured  by  the  oxidation.  So  in 
protoplasm,  various  types  of  energy  may  result  from  the  tear- 
ing down  of  the  complex  substances.  Among  these  are  animal 
heat,  motion,  nervous  energy  and  electrical  energy.  The  living 
body  with  its  protoplasm  is  really  a  mechanism, — an  engine. 

27.  Secretion  and  Excretion. — As  a  result  of  the  constructive  and  destructive 
work  already  mentioned  as  characteristic  of  protoplasm  certain  substances,  not 
themselves  protoplasm,  may  be  produced.     If  these  products  are  of  further  use  in 
the  animal  economy,  they  are  usually  described  as  secretions;  if  they  represent  the 
final  reduction  in  the  process  of  tearing  down,  they  are  called  excretions.     Such 
materials  may  be  deposited  either  within  the  protoplasm  or  at  its  surface.      In  the 
latter  case  it  may  be  deposited  in  a  uniform  sheet  and  produce  a  protective  mem- 
brane (cell  wall).     The  presence  of  such  a  covering  to  the  protoplasm  very  materi- 
ally modifies  all  the  elementary  activities  which  have  been  described. 

28.  Demonstrations. — The  teacher  should  make  microscopic  demonstrations  of 
secretions  and  excretions: — as  starch  grains  formed  in  the  leaves  of  plants;  fat 
in  adipose  tissue;  cell- walls  in  plants;  crystals  in  plant  cells  (see  Botanies);  inter- 
cellular substance  in  cartilage  or  bone. 

29.  Supplementary  Topics  for  Library  Work. — Find  and  examine  some  of  the 
classic  definitions  of  life.     Examine  more  completely  the  theories  of  protoplasmic 
architecture.     In  what  ways  would  the  presence  of  the  cell-wall  bring  about 
modifications  of  the  protoplasmic  activities?     Give  an  account  of  experiments 

2 


I 8  ZOOLOGY 

showing  the  effect  of  some  of  the  more  important  stimuli  on  protoplasm  (as  light, 
heat,  electricity).  What  of  the  external  conditions  are  so  important  as  to  merit 
the  term  "primary  conditions  of  life"?  Why  may  protoplasm  be  described  as 
chemically  unstable?  Compare  oxidation  in  the  protoplasm  with  oxidation  in 
ordinary  combustion. 

30.  Summary. — i.  Scientists  are  not  agreed  whether  life 
is  merely  the  action  of  the  ordinary  chemical  and  physical 
forces  in  connection  with  a  peculiar  substance,  or  represents 
these,  guided  by  a  type  of  energy  of  a  higher  order. 

2.  Protoplasm,  a  chemical  mixture  of  exceeding  complexity 
and  instability,  is  the  "physical  basis  of  life."     Differences 
in  various  living  things  are  probably  due  to  differences  in  the 
chemical  and  physical  structure  of  the  protoplasm  of  which 
they  are  composed. 

3.  Owing  to  the  unstable  character  of  the  protoplasm  it  is 
readily  acted  upon  and  changed  by  external  forces;  and  the 
various  parts  of  the  protoplasm  act  on  each  other  in  such  a 
way  as  to  produce  a  display  of  energy.     The  agents  are  called 
stimuli.     Protoplasm  responds  to  stimuli  because  of  its  irri- 
tability and  contractility.     These  latter  powers  belong  natively 
to  protoplasm  because  of  its  physical  and  chemical  composition. 

4.  Protoplasmic  matter  and  the  materials  which  are  de- 
stroyed in  the  production  of  energy  are  alike  produced  by  the 
assimilation  of  food  substances  into  new  protoplasm.     This 
is  a  most  fundamental  quality. 

5.  Growth  is  increase  of  mass,  following  the  formation  of 
new  substance  by  assimilation.     The  mere  absorption  of  water 
also  results  in  growth.     Growth  leads  naturally  to  reproduction. 

6.  Oxygen  is  one  of  the  chief  agents  by  which  the  unstable 
compounds  in  the  protoplasm  are  made  to  release  their  energy. 
The  breaking  down  of  these  compounds  leaves  unused  mate- 
rials which  must  be  excreted.     Respiration,  which  is  a  term 
applied  to  the  using  of  oxygen  and  the  elimination  of  carbon 
dioxid,  and  excretion  are  thus  seen  to  be  protoplasmic  func- 
tions immediately  connected  with  its  activity. 


CHAPTER  III 

THE  ANIMAL  CELL;  ITS  MORPHOLOGY  AND  PHYSIOLOGY 

31.  Introduction. — In  studying  the  structure  of  organisms 
two  methods  are  open  to  the  student  of  to-day.     He  may 
begin  with -the  whole  adult  individual  and  by  dissection  he  may 
reach  a  knowledge  of  the  constituent  parts, — organs,  tissues, 
cells.     This,  the  analytic  method,  is  the  method  of  history  and 
has  given  us  the  mass  of  details  which  we  have  at-  present. 
On  the  other  hand,  it  is  possible  to  avail  one's   self   of   the 
results  of  such  studies,  to  assume  the  unit  of  structure  which 
is  uniformly  found,   and,   by  a  synthetic  process,  follow  the 
building  up  of  an  organism  from  its  elementary  parts.     This 
is  the  process  which  the  development  of  the  individual  illus- 
trates.    It  has  the  special  advantage  of  emphasizing  the  funda- 
mental unity  of  origin  of  the  organs,   and  the  likenesses  of 
organisms,   and  gives  the  true  significance  of  differentiation 
and  development 

32.  The   Cell. — Having  discussed  in  Chapter   IT   the   sub- 
stance  in   connection   with   which   life   manifests   itself,   it   is 
necessary  to  recall  the  fact  that  the  protoplasm  of  an  organism, 
while  connected  in  various  ways,  is  separated  by  boundaries 
into  unit-masses,  each  mass  having  the  essential  qualities  of 
the  whole.     Each  unit  mass  of  protoplasm  is  called  a  cell.     The 
cell  is  not  to  be  considered  as  the  ultimate  unit  of  structure; 
it  is  itself,  as  we  shall  see,  a  group  of  bodies  which  are  in  turn 
composite.     It  is   thus   to  be  looked   upon   as    an    organized 
structure. 

33.  Cell    Form. — Cells,   unhampered    in   the    direction    of 
growth,  tend  to  assume  a  spherical  form.     Agencies,  both  in- 
ternal and  external,  as  nutritive  processes,  tension,  pressure, 
etc.,  may  modify  this  in  such  a  way  that  almost  any  form 
may    be    found:    polygonal,     flattened,     elongated,     fibrous, 
branched,  etc. 

19 


20 


ZOOLOGY 


34.  Size. — While  ordinary  tissue  cells  are  minute,  there  is 
great  variation  in  the  size  of  cells.  Many  single-celled  in- 
dividuals are  visible  to  the  naked  eye  and  egg-cells  may  be 
several  centimetres  in  diameter;  yet  many  tissue  cells  are  less 
than  .005  millimetre  in  diameter.  Cells  may  be  very  much 


FIG.  4. 


FIG.  5. 


nu. 


niu 


cy...- 


—  •=••  a 


FIG.  4.  Diagram  showing  the  principal  parts  of  the  cell  and  something  of  the  protoplasmic 
architecture  as  it  might  appear  while  living.  «,  alveoli  or  spheres  in  the  foam-work  (see  §18);  c, 
centrosome;  cy,  cytoplasmic  mesh  work,  containing  granules;  nu.,  nucleus;  n,  nucleolus;  v,  vacuole; 
w,  cell  wall. 

FIG.  5-  Diagram  showing  principal  parts  of  the  cell  as  it  appears  when  killed  and  stained.  The 
protoplasm  shows  more  of  a  mesh  work  (cy),  the  spaces  representing  the  alveoli.  /,  formed  sub- 
stance in  alveoli.  Other  letters  as  in  Fig.  4. 

Questions  on  figures  4  and  5. — If  these  cells  are  in  reality  25  /A  in  diameter, 
how  much  are  they  enlarged  in  the  drawing?  (jj.  is  .001  mm.).  Identify  the  various 
structures  referred  to  in  section  35. 

extended  in  one  or  more  directions  without  possessing  great 
bulk.  The  outgrowths  of  nerve  cells  for  example  may  attain  a 
length  of  severaj,  feet,  as  when  the  nerve  fibres  extend  from  the 
trunk  to  the  tips  of  the  toes. 

35.  Structure. — The. following  parts  are  to  be  distinguished 
in  the  typical  cell: — (i)  a  general  cell  substance,  partly  living 
protoplasm,  partly  non-active  matter  both  organic  and  in- 
organic; (2)  usually  a  single  highly  differentiated  nucleus 
which  contains  living  protoplasm  and  is  clearly  demarcated 
from  the  protoplasm  about  it;  (3)  one  or  more  specialized 
bodies  known  as  centrosomes;  (4)  a  cell  wall  or  membrane 
(Figs.  4  and  5). 

The  cell-substance  or  cytoplasm  embraces  that  portion  of 
the  protoplasm  outside  the  nucleus.  This  is  apparently  made 


THE    ANIMAL   CELL  21 

up  of  a  more  dense  portion  which  coagulates  readily  with  certain 
reagents,  and  the  more  fluid  cell  sap  which  is  composed  of  water 
with  sugar,  inorganic  matter,  and  other  substances  dissolved 
in  it.  Suspended  in  the  protoplasm  and  making  a  part  of  it 
may  be  starch  grains,  fats,  crystals,  and  certain  active  bodies 
known  as  plastids,  mitochondria,  etc. 

36.  The    Nucleus. — The    usually    single    nucleus    lies    im- 
bedded in  the  cytoplasm  and  is  ordinarily  separated  from  it 
by  a  thin  membrane.     Nuclei  vary  greatly  in  shape,  size,  and 
degree  of  differentiation.     While  it  is  not  always  possible  to 
find  definite  nuclei  in  all  cells,  it  seems  probable  that  all  cells 
have  nuclear  material  in  one  form  or  another  at  some  stage 
of   their   history.     The   internal   structure   of   the   nucleus   is 
equally  as  complex  as  that  of  the  cytoplasm,  having  both  living 
and   non-living   portions.     It   usually   consists   of   a   network 
of  threads   (chromatin)   readily   stained  by  certain  dyes.     In 
the   meshes   of   this   a  less   easily,    stainable    material   occurs 
(achromatin) ,   a  portion  at  least  of  which  is  active.     One  or 
more  deeply  stainable  bodies,   called  nucleoli,   usually  occur, 
the  real  character  of  which  is  difficult  to  estimate. 

37.  Centrosomes   or   Centrospheres. — These   bodies   lie  in 
the  cytoplasm  but  are  closely  related  to  the  nucleus,  and  ap- 
pear to  have  an  important  place  in  certain  phases  of  cell  activity 
(see  "cell  division,"  §41).     The  centrosphere  is  a  clear  space 
about  the  centrosomes. 

At  such  times  the  cytoplasmic  elements  radiate  from  the 
centrosomes  in  a  very  characteristic  way  (Fig.  8,  c).  The 
influence  extends  even  into  the  nucleus  and  is  accompanied  by 
a  rearrangement  of  the  chromatic  elements.  The  origin  of  the 
centrosomes  is  still  a  matter  of  disagreement.  The  centre- 
spheres  are  often  spoken  of  as  attraction  spheres  from  the  fact 
that  they  appear  to  exert  an  attractive  influence  upon  certain 
portions  of  the  protoplasm. 

38.  Cell  Wall. — A   cell   membrane   usually   surrounds   the 
protoplasm.     It  may  be  a  non-living  organic  secretion,  or  may 
consist  of  metamorphosed  or  altered  protoplasm  in  connection 
with  such  secretion.     The  wall  is  protective  and  supportive 


22 


ZOOLOGY 


in  function,  and  varies  much  in  thickness,  resistance,  etc. 
Animal  cells  as  a  rule  are  not  provided  with  such  well  de- 
veloped and  resistant  walls  as  are  plant  cells. 

39.  Cell  Functions. — Since  the  cell  is  only  a  definite  unit 
mass  of  protoplasm,  its  functions  are  in  general  those  which 
have  already  been  described  as  protoplasmic  functions.  They 

FIG.  6. 


C 


m. 


PIG.  6.  Modes  of  cell  reproduction.  A,  B,  and  C,  stages  in  the  reproduction  of  the  Infusorian, 
Colpoda,  by  the  breaking  up  of  the  protoplasm  to  form  numerous  cells.  A.  encysted  stage;  B 
protoplasm  escaping,  spores  partly  formed;  C,  spores  completely  separated  (adapted  from  Rhum 
bier);  D,  budding  in  Chlamydomyxa,  a  lowly  Rhizopod.  The  bud  is  finally  constricted  off  from 
the  mother  cell,  b,  bud;  cie.t  cell  wall;  m,  mother  cell;  n,  nuclear  matter;  s,  spores. 

Questions  on  the  figure. — Compare  the  process  and  the  results  of  the  two  modes 
of  cell  reproduction  shown  in  this  figure.  Can  you  describe  the  fate  of  the 
"mother"  cell  in  the  two  cases? 

are  merely  localized  within  the  cell.  The  cell  wall  when  present 
would  naturally  modify  and  limit  in  important  ways,  the  more 
active  protoplasmic  functions,  especially  motion.  In  such  cases 
the  independent  motion  characteristic  of  so  many  cells  must  be 
accomplished  by  special  devices.  These  frequently  take  the 
form  of  cilia  or  flagella,  which  are  thin  protoplasmic  projec- 
tions penetrating  the  cell  wall  and  used  after  the  manner  of 
oars.  Locomotion  of  cells  is  not  confined  to  single-celled  or- 
ganisms, but  is  found  in  many  cells  of  the  higher  animals  and 
plants — as  colorless  blood  cells,  sexual  cells,  etc.,  which  have 


THE   ANIMAL   CELL  23 

a  distinct  motion  of  their  own.  The  muscle  cells  of  higher 
animals  possess  the  power  of  contraction  and  motion  in  a  high 
degree. 

40.  Reproduction. — The  cell  grows  as  a  result  of  the  nu- 
tritive processes  and  reaches  the  limits  of  size  determined  by 
its  special  conditions.     The  internal  and  external  conditions 
together  constitute  a  stimulus  to  the  breaking  up  or  division  of 
the  protoplasmic  unit.     This  may  occur  (i)  by  the  irregular 
breaking  up  of  the  protoplasm  into  numerous  masses,  each 
of  which  has  the  essential  qualities  of  the  whole  (Fig.  6,  A,  and 
£);   (2)   by  budding,  in  which  a  process,  or  several  processes, 
appears  on  the  cell,  develops  into  bodies  like  the  original  cell,  and 
finally  becomes  separate  from  it  (Fig.  6,  D}\  (3)  by  division, 
in  which  there  is  a  division  of  the  original  protoplasm  into 
two  essentially  equal  parts.     In  the  last  case  neither  of  the 
cells  can  be  considered  the  parent  of  the  other. 

41.  Cell  Division. — Cell  division  may  be  effected  in  either 
of  two  ways,   (a)  by  direct  or  amitotic  division,  in  which  the 


FIG.  7.  Direct  cell  division  (Amce&a).  A,  active  specimen  with  pseudopodia;  B,  becoming 
spherical  preliminary  to  division;  C,  beginning  of  elongation  and  constriction;  D,  later  stage;  JB, 
daughter  cells  forming  pseudopodia.  ec.  clear  ectoplasm;  en,  granular  endoplasm;/,  food  vacuole; 
«,  nucleus;  pst  pseudopodium;  v.  pulsating  vacuole. 

Questions  on  the  figure. — Why  is  this  properly  called  direct  division?  What 
structures  are  divided?  Are  the  resulting  halves  exactly  or  merely  roughly  equal, 
apparently?  Do  you  see  any  possible  gain  to  the  organism  in  such  a  division  as 
this? 


24  ZOOLOGY 

nucleus  and  cell  merely  constrict  into  two  nearly  equal  parts 
(Fig.  7);  and  (b)  indirect  or  mitotic  division.  The  latter  is 
the  usual  method  and  is  very  complicated.  By  means  of  it  a 
very  even  division  of  the  substances  and  structures  of  the 
nucleus,  especially,  seems  to  be  secured. 

The  more  striking  stages  in  the  process  as  it  usually  occurs 
are  outlined  in  the.  text  and  figures  which  follow.  The  centro- 
somes  and  nucleus  will  be  seen  to  be  especially  active.  Such 
a  description  is  only  typical  of  the  wonderful  series  of  events. 
There  are  numerous  variations  from  this  in  different  organisms. 

1.  In  the  quiescent  or  resting  stage  the  structural  elements 
are  distributed  in  the  way  characteristic  of  the  particular  cell 
under  examination  (Fig.  8,  A) .     Most  cells  are  in  this  stage  when 
examined. 

2.  When  division  is  about  to  take  place,   the  chromatin 
elements  in  the  network  of  the  nucleus  assume  the  appearance 
of  a  coil  or  tangle  of  thread  (Fig.  8,  B).     The  nuclear  membrane 
often  breaks  up  at  this  time. 

3.  The  centrosome  divides  and  the  halves  migrate  to  op- 
posite poles  of  the  nucleus,  and  from  them  as  centres  radiations 
pass  into  the  cell  body  in  all  directions.     Across  the  nucleus, 
from  one  centrosphere  to  the  other,  thread-like  lines  extend, 
producing  the  appearance  of  a  spindle   (Fig.   8,   C,   sp).     In 
the  meantime  the  coil  of  chromatin  has  been  unraveled  and  has 
broken   up   into   a   definite   number   of   pieces    (chromosomes) 
which  often  form  into  V-shaped  loops.     After  certain  evolutions, 
under  the  influence  of  the  centrospheres  apparently,  these  loops 
come  to  lie  in  the  equatorial  plane  of  the  spindle,  the  apices 
of  the  loops  pointing  toward  the  centre  of  the  nucleus.     This 
is  called  the  astroid  stage  (Fig.  8,  C).     The  process  up  to  this 
point  is  known  as  the  pro  phase  or  preparation  stages. 

4.  Each  of  the  chromatin  loops  next  splits  longitudinally 
into  two.     This  is  the  metaphase  or  middle  stage  (Fig.  8,  D). 

5.  Each  of  these  halves  now  begins  to  move  toward  its 
appropriate  pole  or  centrosome  (Fig.  8,  E).     As  these  half -loops 
leave  the  equator  and  collect  about  the  poles  they  give  rise  to 
a  double-star  appearance  or  diastroid  stage  (Fig.  8,  F).     This 
is  the  anaphase. 


THE    ANIMAL   CELL  25 

6,  The  loops  of  chromatin  collected  at  each  pole  are  re- 
constructed into  a  coil  which  then  passes  gradually  into  the 
resting  stage  at  the  new  position,  a  membrane  is  formed,  and 
the  daughter  nucleus  is  complete.  The  nuclear  spindle  dis- 

FIG.  8. 


FIG.  8.  Indirect  or  mitotic  division  (diagrammatic);  A,  resting  mother  nucleus;  B,  coil  stage, 
with  the  centrosomes  separating;  C,  D  (metaphase),  and  E,  stages  in  the  division  of  the  chromo- 
somes (the  cell  wall  is  not  shown  in  these  three  drawings);  F,  diastroid  (anaphase)  stage;  G  and  H 
show  the  return  of  the  daughter  nuclei  to  the  coil  and  to  the  resting  condition,  and  division  of  the 
cytoplasm,  and  the  formation  of  the  dividing  wall:  c,  centrospheres;  cl,  chromatin  coil;  chr,  chro- 
mosomes; nu.,  nucleus;  »,  nucleolus;  sp,  nuclear  spindle;  w,  cell  wall. 

Questions  on  the  figure. — What  structures  possessed  by  the  original  cell  are 
divided  in  this  process  ?  In  what  order  ?  Why  is  this  termed  ' '  indirect "  division  ? 
Which  is  the  more  common,  the  direct  or  the  indirect?  Can  you  see  any  special 
gain  secured  by  this  method?  Describe  the  behavior  of  the  nucleolus  and  the 
nuclear  membrane  by  comparing  this  with  other  figures  in  reference  books. 


26  ZOOLOGY 

appears,  the  radial  appearance  about  the  centrosomes,  and 
even  the  centrosome  itself,  may  disappear  or  become  incon- 
spicuous. 

7.  Accompanying  or  following  the  last  nuclear  changes  the 
cytoplasm  may  have  become  constricted  into  two  masses,  or 
separated  by  the  formation  of  a  wall  perpendicular  to  the  axis 
of  the  spindle  (Fig.  8,  G,  H).  The  daughter  cells  may  separate 
or  remain  united.  These  final  stages  are  known  as  the  telo- 
phase.  There  are  other  protoplasmic  bodies  in  the  cytoplasm 
which  seem  to  divide,  though  not  so  accurately  as  the  nucleus, 
into  two  sub-equal  portions.  These  are  variously  named 
plastids,  mitochondria,  etc.  The  important  point  is  that 
there  seems  to  be  a  definite  tendency  to  get  equal  division  of  the 
cell  materials. 

Cell  division  is  at  the  beginning  of  all  the  complexities 
of  structure  found  in  the  higher  forms  of  animals.  Each  sexually 
produced  organism  commences  life  as  a  single  cell,  from  which 
the  adult  is  formed  by  cell-division,  and  the  clinging  together 
of  the  daughter  cells. 

42.  Functions  of  the  Nucleus  and  Centrosomes. — While  we 
can  follow  some  of  the  externals  of  the  various  cell  activities, 
the  manner  of  their  occurrence  and  their  causes  are  in  the 
greatest  obscurity.  We  are  not  able  to  say  just  what  part  is 
performed  by  the  different  structures  involved.  It  is  hazardous 
to  say  that  one  structure  is  more  important  than  another;  yet 
it  seems  to  be  proven  that  the  nucleus  is  quite  essential  in 
cells  which  possess  nuclei,  for  the  proper  performance  of  even 
the  ordinary  nutritive  functions.  Some  of  the  unicellular 
animals  may  be  artificially  mutilated  in  such  a  way  that  the 
lost  parts  may  be  regenerated  and  the  normal  form  restored. 
A  relatively  small  piece  of  the  Protozoan,  Stentor,  for  example, 
can  reproduce  the  whole,  if  a  portion  of  the  nucleus  be  present. 
A  much  larger  piece  without  nuclear  material  is  wholly  unable 
to  regenerate  lost  parts,  and  even  seems  unable  to  control  or 
exercise  the  ordinary  assimilative  functions.  The  phenomena 
of  indirect  cell  division  show  that  activity  on  the  part  of  the 
centrosomes  and  nucleus  precedes  that  of  the  cytoplasm. 


THE   ANIMAL   CELL  2*J 

Experiments  also  show  that  the  division  of  the  cytoplasm  may 
be  checked  or  interrupted  by  external  influences  without 
interfering  with  the  division  of  the  nucleus.  On  the  other  hand 
nuclei  separated  from  cytoplasm  are  incapable  of  continuing 
their  functions.  We  are  at  least  safe  in  saying  that  these  three 
bodies,  the  centrosome,  the  nucleus,  and  the  cytoplasm  act  as 
intracellular  stimuli  upon  each  other,  and  that  all  are  important 
in  the  work  of  the  cell.  During  nuclear  division  there  is  prob- 
ably increased  interchange  between  the  cytoplasm  and  nucleo- 
plasm.  The  breaking  down  of  the  nuclear  membrane  facili- 
tates this. 

43.  Exercises  for  Library  and  Laboratory. — The  teacher  should  by  all  means 
secure  preparations  of  properly  stained  cells  showing  the  principal  structures;  also 
some  of  the  stages  of  cell  division  (see  Appendix:  Laboratory  Suggestions). 

What  are  chromosomes?  In  what  respects  and  to  what  extent  do  nuclei  differ? 
What  is  meant  by  the  "cell-doctrine " ?  Give  an  outline  of  its  history.  What  are 
mitochondria?  Compare  the  various  series  of  figures  in  your  library  illustrating 
the  stages  of  cell  division. 

44.  Summary. 

1.  The  cell  may  be  considered  as  the  unit  of  structure,  and 
is  to  be  defined  as  a  "nucleated  mass  of  protoplasm  with  or 
without  a  cell  membrane." 

2.  The  cell  may  also  be  considered  the  unit  of  function,  in 
the  sense  that  it  embodies  all  vital  functions  in  epitome. 

3.  The  structure  of  the  typical  cell  may  be  outlined  as 
follows : 

(a)  Cell  body 

Cytoplasm — living. 
Cytolymph — non-living,  fluid. 
Metaplasm — non-living,  solid. 

(6)  Nucleus: 

Nucleoplasm — living. 
Chromatin. 
Achromatin. 

Nucleolymph — non-living,  fluid. 
Metaplasm — non-living,  solid. 
[Protoplasm  =  Cytoplasm  +  nucleoplasm.] 


28  ZOOLOGY 

(c)  Centrosome. 

(d)  Cell  wall. 

4.  In  addition  to  the  general  functions  of  protoplasm  which 
cells  possess  we  need  to  consider  in  connection  with  cells  the 
additional  functions  : 

(a)  Locomotion. 
(6)  Reproduction. 

5.  Reproduction    of    cells    occurs    by    fragmentation,    by 
budding,  and  by  division.     Division  may  be  either  direct  or 
indirect. 

6.  The   following   diagram,    adapted   from   Flemming   will 
serve  to  represent  the  stages  in  indirect  division: 


One  mother  nucleus.  Two  daughter  nuclei 

(a)  Resting  stage.  Resting  st 

(&)  Coil  stage.  1  =prophase  Coil  stage  (f. 

)  Astroid  stage.  )  Astroid  stage  (e. 

(d)  Division  of  chromatin  loops  =  metaphase  (d.  / 


7.  The  important  effect  of  this  complicated  process  is,  ap- 
parently, to  secure  an  equal  division  of  the  nuclear  elements 
for    the    daughter    cells.     The    cytoplasmic    elements    in    the 
daughter  cells  may  be  strikingly  unequal. 

8.  The  exact  relations  of  the  various  structures  in  the  cell 
are  not  known.     They  cannot  be  understood  until  the  chemical 
and    physical    nature    of   living   protoplasm   is   known.     The 
cytoplasm,  the  centrosomes,  and  the  nucleus  seem  to  act  as 
stimuli  to  one  another,  in  assimilation,  growth,  and  division. 


CHAPTER  IV 

FROM  THE  SIMPLE  CELL  TO  THE  COMPLEX  ANIMAL 

45.  The  Individual  as  a  Cell -composite. — In  the  simplest 
animals,  as  the  Protozoa,  the  individual  consists  of  a  single 
cell,  and  the  life  history  of  the  individual  animal  is  such  as 
has  already  been  seen  to  belong  to  the  cell  (Chapter  III).  In 
such  an  individual  one  cannot  speak  of  organs  in  the  ordinary 
sense,  for  organs  as  we  shall  see  are  made  up  of  cells  bound 
together  in  the  doing  of  certain  work.  Yet  it  is  important  to 
remember  that  there  are  none  of  the  necessary  duties  of  life, 
such  as  getting  food,  digesting  it,  breathing,  moving,  repro- 
ducing, and  the  like,  which  are  not  well  done  by  these  simple 
one-celled  animals.  The  many-celled  animals  agree  with  the 
simpler  ones  in  that  they  too  start  life  as  single  cells  appar- 
ently quite  as  simple  as  the  one-celled  animals  themselves. 
When  the  cells  divide,  however,  the  daughter  cells  do  not 
separate  as  in  the  Protozoa,  but  form  a  mass  of  cells  by  cling- 
ing together.  Owing  both  to  internal  and  external  forces 
the  cells  in  the  mass  do  not  long  remain  alike,  but  soon  show 
such  differences  among  themselves  as  serve  as  the  basis  for 
the  great  variety  of  structures  found  in  the  bodies  of  the  higher 
animals.  The  change  from  the  simple  cell  to  the  complex 
condition  in  the  adult  animals  is  not  a  sudden  one,  but  takes 
place  very  gradually  and  the  work  which  was  formerly  done 
by  the  single  cell  is  divided  up  among  the  groups  of  different 
cells  composing  the  body  The  division  of  the  work  to  be 
done  makes  possible  and  necessary  the  specializing  of  certain 
cells  to  do  each  part  of  it,  and  the  differentiation  of  structures 
makes  it  possible  to  do  each  separate  task  better  than  before. 
Thus  division  of  labor  and  differentiation  of  parts  go  hand  in 
hand  as  we  pass  from  the  simple  to  the  complex  animals. 

29 


30  ZOOLOGY 

46.  The  Fertilized  Ovum  the  Starting  Point— In  speak- 
ing of  the  development  of  the  adult  animal  from  the  simpler 
condition  of  the  single  cell  it  is  necessary  to  remember  that 
this  cell,  which  has  the  power  of  giving  rise  to  a  complex  in- 
dividual and  is  called  a  fertilized  ovum,  has  a  history  that  is 
very  important.     The  fertilized  ovum  represents  the  union  of 
two  distinct  cells,  known  as  germ  or  sexual  cells,  which  are 
ordinarily  quite  different  in  appearance  and  produced  by  dif- 
ferent kinds  of  individuals,  males  and  females.-   Both  classes 
of  cells  may  be  produced  by  the  same  individual.     This  union 
does  not  produce  a  double  cell,  but  the  parts  of  each  seem  to 
fuse  with  those  of  the  other  in  a  very  complete  way. 

47.  The  Ovum. — The  germ  cell  produced  by  the  female  is 
known  as  the  ovum,   and  is   typically  a  spherical   cell  with 


FlG.  Q.  Types  of  ova.  A,  primitive  amoeboid  ovum  of  Sponge;  B,  semi-diagrammatic  figure 
of  spherical  ovum  of  Sea-urchin  in  which  the  yolk  is  uniformly  distributed;  C,  figure  of  a  spherical 
ovum  (such  as  may  be  found  in  some  Worms  or  in  the  Frog)  in  which  the  yolk  tends  to  collect  at 
one  pole,  p.p.,  and  the  nucleus  and  protoplasm  at  the  other,  a.p.;  m,  micropyle;  nu,  germinal  vesicle 
(nucleus);  »,  germinal  spot  (nucteolus);  y,  yolk  spheres. 

Questions  on  the  figure. — What  are  the  points  of  agreement  in  these  three  ova? 
The  chief  points  of  contrast?  What  is  the  function  of  the  micropyle?  Is  a 
micropyle  always  present  in  ova?  Why  are  the  poles  of  the  ovum  appropriately 
called  active  and  passive  ? 

abundant  nourishment  and  inactive  as  compared  with  the 
male  cell.  It  often  has  an  especially  well-developed  cell- 
covering.  Its  nucleus  is  sometimes  called  the  germinal  vesicle 
(Fig.  9).  The  ovum  must  be  distinguished  from  what  is 
popularly  known  as  an  egg.  The  latter  term  is  loosely  used 
to  describe  the  fertilized  ovum  more  or  less  developed,  to- 
gether with  its  nutritive  and  protective  coats  such  as  occur 


FROM   SIMPLE   CELL   TO   COMPLEX  ANIMAL  31 

around  the  eggs  of  birds  and  reptiles.  Ova  differ  very  greatly 
in  size.  The  largest  are  found  among  the  birds.  The  ' '  yellow ' ' 
of  these  eggs  represents  the  real  size  of  the  ovum.  Variations 
in  size  are  due  not  so  much  to  a  difference  in  the  amount  of 
protoplasm  as  to  a  varying  amount  of  food  or  yolk  in  the 
cell.  The  food  may  be  uniformly  distributed  throughout 
the  ovum,  mingled  with  the  protoplasm,  or  it  may  collect  at 
one  pole,  forcing  the  active  protoplasm  to  occupy  the  other 
pole  (Fig.  9,  C).  The  yolk  furnishes  food  to  the  young  indi- 
vidual or  embryo  in  its  early  development,  that  is,  during  cell- 
division,  before  it  can  get  food  for  itself. 

48.  The   spermatozoon   or   male   element   is   ordinarily   in 
striking  contrast  to  the  female.     It  is  typically    very  small* 


-n. 


PIG.  10.  Types  of  spermatozoa.  A,  from  the  round  worm  (Ascaris)  with  a  cap,  somewhat 
amceboid;  B,  from  the  Crayfish,  with  numerous  projections;  C,  from  Frog;  D,  from  Sea-urchin.. 
h,  head;  m,  middle  piece;  n,  nucleus;  t,  tail  or  flagellum.  The  head  is  chiefly  nucleus.  ». 

Questions  on  the  figure. — What  are  the  chief  points  of  similarity  and  dissimi* 
larity  in  these  spermatozoa?  How  do  they  agree  with,  and  how  differ  from,  the 
ova  in  Fig.  9?  How  do  they  differ  from  the  average  cell?  What  parts  of  the: 
structure  of  typical  cells  are  believed  to  be  represented  in  the  sperm  cells? 

active,  and  with  thin  protoplasmic  projections  (Fig.  10).  Struc- 
turally, the  typical  spermatozoon  consists  of  a  "head"  piece,, 
a  middle  piece,  and  a  ' '  tail ' '  or  flagellum.  The  head  is  composed 
chiefly  of  the  chromatic  material  of  the  nucleus.  A  delicate- 
covering  of  cytoplasm  envelops  the  head  and  is  drawn  out  into* 


32  ZOOLOGY 

the  projection  known  as  the  tail  (Fig.  10,  D).     The  middle  piece 
contains  the  centrosome  of  the  male  cell. 

49.  Early    History    of     Ova    and     Sperm. — We    formerly 
thought  of  ova  and  sperm  as  actually  formed  by  the  body  of  the 
parent.     We  now  know  that  these  germ  cells  are  set  apart  early 
in  the  history  of  each  individual,  before  the  body  is  developed. 
They  of  all  the  cells  retain  their  primitive  undifferentiated 
nature.     The  body  cells  become  different  in  appearance  and 
powers, — as  nerve  cells,  muscle  cells,  bone  cells,  and  the  like. 
The  body  does  not  produce  the  germ  cells ;  it  merely  houses  and 
nourishes  them  during  their  development.     The  body  cells  and 
the  germ  cells  develop  side  by  side,  each  influencing  the  other  in 
their  development. 

After  the  first  putting  aside  of  the  primordial  germ  cells  in 
the  early  life  of  the  individual,  the  body  cells  have  a  period  of 
rapid  division  and  growth.  During  this  time  the  germ  cells  are 
relatively  quiet.  As  the  parent  animal  becomes  mature  the  pri- 
mordial germ  cells  enter  upon  a  period  of  activity  by  which  they 
produce  such  ova  and  sperm  as  are  described  in  the  preceding 
sections.  This  period  of  activity  of  the  germ  cells  shows  three 
stages:  (i)  a  period  of  increase  of  the  primordial  germ  cells 
(oogonia  and  spermatogonid)  by  division;  (2)  a  period  in  which 
the  last  descendents  of  these  divisions  enlarge  (becoming  pri- 
mary oocytes  and  spermatocytes) ;  and  (3)  a  period  of  maturing 
or  perfecting  these  (into  ova  and  spermatozoa) .  These  stages  are 
suggested  in  Fig.  n. 

50.  Maturation  of  Ova  and  Sperm. — This  third  stage,  the 
maturation  or  ripening  of  the  germ  cells,  shows  us  some  of  the 
most  remarkable  happenings  to  be  found  in  all  biology.     Both 
ova  and  sperm  when  ripe  contain,  as  a  rule,  just  one-half  as 
many  chromosomes  in  their  nuclei  as  are  found  in  the  primordial 
germ  cells,  or  in  the  body  cells,  of  the  species  to  which  they  be- 
long.    Somewhere  in  the  history  of  the  egg  and  sperm  there  is 
therefore  a  reduction  division  of  chromatic  material  in  the  nu- 
cleus.    It  does  not  always  take  place  in  just  the  same  way,  but 
the  following  will  illustrate  the  process.     The  student  should 


MALE 


FROM   SIMPLE   CELL   TO   COMPLEX  ANIMAL 

FlG.   II. 
PRIMORDIAL  GERM  CELLS 


33 


FEMALE 


%£?          «j»'  UNIVA 

/'  l\    SPERMAT 


LENT  CHROMOSOMES  i2X> 

SPERMATOGONIA  OOGONIA 


BIVALENT  CHROMOSOMES  (X) 
PRIMARY  PRIMARY  OOCYTE  /    */> 

SPERMATOCYTE 

SECONDARY 
X)SPERMATIDS  OOCYTES 

j-.  i*-S        UNIVALENT  CHROMOSOMES  iX) 


I     MULTIPLICATION  PERIOD 
(MANY  GENERATIONS; 


GROWTH  PERIOD 


III.    MATURATION  PERIOD 


V.    FERTILIZATION  PERIOD 


V.    CLEAVAGE  PERIOD 


VI.  PERIOD  OF  BODY-FORMATION 
BY  CELL  DIVISION.  AND  SEG- 
REGATION OF  GERM  CELLS 


PRIMORDIAL  GERM  CELL 

FIG.  ii.  A  diagram  of  the  germ  cell  cycle,  suggesting  the  changes  by  which  the  primordial 
germ  cells  in  the  body  give  rise  to  the  mature  eggs  and  sperm;  fertilization;  cleavage;  and  the  segre- 
gation of  new  primordial  germ  cells  as  the  new  body  is  developed.  (See  §§40-52,  60.)  The  full 
number  of  chromosomes  in  the  cells  of  the  body  is  2x.  The  half  number  found  at  certain  stages  is 
indicated  by  x.  The  large  gastrula  at  the  bottom  of  the  figure  is  to  illustrate  the  early  differentia- 
tion of  germ  and  body  cells. 

Questions  on  the  figure. — Trace  the  changes  of  the  chromosomes  through  the 
whole  series  of  stages.  The  solid  black  is  intended  to  emphasize  the  potential 
germ  cells.  Why  are  all  the  cells  of  the  two  and  four-celled  stages  represented  in 
black?  Why  are  the  chromosomes  in  the  primary  oocyte  and  spermatocyte  said 
to  be  bivalent?  What  is  meant  by  univalent?  What  does  "continuity  of  germ- 
plasm"  mean? 
3 


34 


ZOOLOGY 
FIG.  12. 


MALE 


FEMALE 


^Jy)]       SPERMATOGONIUM  UNIVALENT  CHROMOSOMES  <2X) 


CHROMOSOMES  UNITE  IN  PAIRS 

BIVALENT  (X)  PRIMARY  OOCYTE' 

PRIMARY  SPERM ATOCYTE 


MATURATION    DIVISION 


SECONDARY        UNIVALENT  CHROMOSOMES  (Xj          SECONDARYJDOCYTE 
SPERMATOCYTF 


II     MATURATION  DIVISION 


8. UNION  OF  NUCLEI 


lO,  CLEAVAGE 


FIG.  12.  A  diagram  to  show  the  central  place  of  the  fertilized  ovum,  and  the  important  stages 
leading  up  to  it  together  with  those  following  it.  Compare  with  Pig.  n.  The  male  chromosome 
are  figured  black  and  the  female  light,  throughout.  Compare  descriptive  matter  in  text. 

Questions  on  the  figure. — In  what  respects  is  the  maturation  of  the  ovum  and 
of  the  sperm  similar?  In  what,  different?  What  is  the  probable  meaning  of  the 
pairing  of  the  chromosomes  (synopsis)  in  the  early  maturation  stages.  How  does 
this  first  division  of  chromosomes  differ  from  the  ordinary  divisions.  Why  may 
fhe  polar  bodies  be  called  abortive  eggs?  What  is  accomplished  by  the  union  of 
the  sperm  and  ovum  nuclei?  In  cleavage  the  chromosomes  from  both  parents 
seem  to  be  distributed  equally  to  all  the  cells  of  the  body.  What  is  the  significance 
to  this? 


FROM   SIMPLE   CELL   TO   COMPLEX   ANIMAL  35 

follow  closely  the  figures  n  and  12,  in  order  to  get  the  steps 
described. 

When  the  full-grown  primary  oocyte  and  spermatocyte  are 
ready  to  divide  at  the  beginning  of  the  third  stage,  the  chromo- 
somes in  a  given  nucleus  unite  and  fuse  in  pairs.  Thus  when 
division  begins  there  is  only  one-half  the  usual  number,  but  each 
chromosome  is  of  double  value  (bivalent).  As  the  primary  cells 
divide  into  the  secondary  oocytes  and  spermatocytes,  the  bival- 
ent chromosomes  behave  as  single  chromosomes  in  ordinary 
nuclear  division  and  split,  and  one-half  of  each  bivalent  chro- 
mosome passes  to  each  daughter  nucleus.  In  this  way  each 
secondary  oocyte  or  spermatocyte  contains  one-half  the 
usual  number  of  single  (or  univalenf)  chromosomes.  By  this 
device  of  first  uniting  and  then  separating  whole  chromosomes 
instead  of  splitting  the  single  chromosomes  into  halves,  as 
occurs  in  ordinary  nuclear  divisions,  an  actual  reduction  of  the 
number  of  chromosomes  is  secured.  Furthermore  it  may  be, 
while  these  chromosomes  are  fused,  there  is  an  actual  interchange 
and  modification  of  the  substance  of  each. 

The  secondary  cells  divide  once  more  in  forming  the  mature 
eggs  and  sperm.  In  this  last  division  the  chromosomes  simply 
split  as  they  do  in  ordinary  indirect  nuclear  divisions. 

In  all  these  steps  in  the  division  of  nuclear  material  the  his- 
tory of  ova  and  sperm  is  essentially  the  same.  There  are, 
however,  very  striking  differences  in  the  way  the  cytoplasm,  and 
the  cell  as  a  whole,  behaves.  In  the  history  of  the  sperm  cells 
the  protoplasm  divides  equally  in  both  divisions,  and  thus  each 
primary  spermatocyte  produces  four  small  spermatids.  Each 
spermatid  gradually  changes,  without  further  division,  into  a 
spermatozoon. 

In  the  divisions  of  the  primary  oocyte,  on  the  contrary,  one 
of  the  daughter  nuclei  enters  a  kind  of  bud  in  the  wall  of  the  cell 
and  carries  very  little  cytoplasm  with  it.  It  is  clearly  an  abor- 
tive cell,  and  is  called  a  polar  body.  The  second  nuclear  division 
results  in  another  abortive  egg-cell  or  polar  body.  The  first 
polar  body  sometimes  divides  into  two.  All  the  polar  bodies 
finally  disintegrate.  Thus,  since  one  of  the  nuclei  has  almost  all 
the  cytoplasm  and  the  others  almost  none  at  all,  there  is  only  one 


36  ZOOLOGY 

mature  well-nourished  egg  as  the  result  of  the  two  divisions.  In 
the  formation  of  the  sperm  there  are  usually  four  perfect  cells 
arising  from  each  spermatocyte. 

51.  Fertilization. — The  union  of  a  sperm  cell  with  an  ovum 
constitutes  the  act  of  fertilization.  Often  there  is  a  special 
aperture  (micro  pyle)  in  the  outer  egg -membrane  through  which 
the  spermatozoon  finds  entrance.  Usually  only  one  sperm  cell 
gains  admission  to  the  interior  of  the  ovum,  whether  by  way  of 
the  micropyle  or  through  the  unmodified  membrane.  Changes 
normally  occur  in  the  membrane  as  soon  as  one  sperm  enters,  by 
which  all  others  are  excluded.  In  eggs  which  have  been  kept  too 
long  or  subjected  to  unfavorable  conditions,  the  response  of  the 
membrane  may  not  be  so  quickly  effected  and  multiple  fertiliza- 
tion may  occur.  Multiple  fertilization  occurs  normally  in  some 
species.  Such  fertilizations  may  produce  monstrosities.  The 
sperm  cell  may  enter  the  egg  even  before  the  polar  bodies  are 
formed;  or  it  may  enter  after  maturation  is  completed.  It 
brings  into  the  egg  the  nucleus,  the  centrosome  and  a  very  small 
amount  of  cytoplasm.  It  at  once  organizes  itself  as  a  second 
nucleus  of  the  egg  and  is  nourished  by  its  substance.  The  sperm 
nucleus  and  the  egg  nucleus,  each  carrying  one-half  the  full  num- 
ber of  chromosomes  of  the  species,  now  draw  together  and  organize 
into  a  new  nucleus.  Thus  is  formed  the  first  segmentation  nucleus, 
and  the  egg  is  fertilized.  With  the  addition  of  the  chromosomes 
in  the  male  nucleus  the  fertilized  ovum  contains  the  same 
number  of  chromosomes  as  before  maturation,  which  in  each 
species  of  animals  is  a  constant  number.  It  appears  that 
fertilization  restores  to  the  female  cell  essentially  what  it  lost  min 
the  process  of  maturation,  and  in  addition  stimulates  it  to  active 
nuclear  and  cytoplasmic  division  as  indicated  in  the  next 
paragraph. 

52.  Segmentation  or  Cleavage. — Following  shortly  upon 
fertilization,  if  conditions  are  favorable,  ordinary  mitotic  nuclear 
division  begins  and  the  ovum  divides  promptly  into  2,  4,  8,  16, 
etc.,  cells  (blastomeres).  In  these  divisions  the  chromosomes 
split,  and  one-half  of  each  chromosome  brought  in  by  the  sperm 
nucleus  and  one-half  of  each  furnished  by  the  egg-nucleus  go  to 


FROM   SIMPLE    CELL   TO    COMPLEX   ANIMAL  37 

each  daughter  nucleus.  This  is  continued  in  later  divisions,  and 
in  this  way  every  cell  of  the  body  gets  its  chromatic  material 
equally  from  the  father  and  the  mother.  The  resulting  cells  be- 
come smaller  and  smaller  with  each  division,  since  the  whole  egg- 
mass  does  not  increase  appreciably  in  size  meanwhile. 

The  first  three  cleavage  planes  are  usually  perpendicular  to 
each  other.  Their  position  is  much  modified,  however,  by 
the  presence  of  food  or  yolk  substance  in  the  egg.  The  yolk  in 
general  retards  cleavage.  If  the  yolk  is  in  small  quantity  and  is 
uniformly  distributed  through  the  egg,  the  blastomeres  will  be 
about  equal  in  size  (Fig.  13,  A),  and  will  continue  to  divide  with 
practically  equal  promptness.  If  there  is  much  of  the  yolk 
it  is  not  likely  to  be  uniformly  distributed.  Under  the  influence 
of  gravity  and  internal  forces,  the  yolk  is  likely  to  collect  at  the 
lower,  and  the  protoplasm  and  nucleus  at  the  upper,  pole  of  the 
ovum  (Fig.  13,  B,  C).  The  protoplasmic  pole  is  known  as  the 
active  or  formative  pole,  and  the  lower  as  the  passive  or  nutritive 
pole.  The  polar  bodies  are  normally  freed  at  the  formative  pole. 
Under  these  circumstances  the  blastomeres  at  the  nutritive  pole 
are  larger  and  divide  less  rapidly  than  those  in  which  the  proto- 
plasm is  in  excess.  If  the  yolk  is  excessive  in  amount  that  por- 
tion of  the  ovum  in  which  it  collects  may  be  totally  prohibited 
from  dividing  as  will  be  seen  in  Fig.  13,  C.D. 

53.  Forms  of  Segmentation. — The  conditions  suggested  above  give  rise  to  the 
following  classes  of  segmentation. 

A.  Total  segmentation. 

I.  Equal:  in  which  there  is  little  yolk  material,  and  that  is  well  distributed. 
(Illustrated  in  most  of  the  lower  invertebrates  and  mammals.)     Fig. 

13,4. 

II.  Unequal:  in  which  there  is  a  moderate  amount  of  yolk  which  accumulates 
at  the  passive  pole.  The  cells  at  the  active  pole  are  more  numerous  and 
smaller  than  at  the  passive.  (Illustrated  in  many  mollusks  and  in  the 
Amphibia.)  Fig.  13,  B. 

B.  Partial  segmentation. 

I.  Discoidal:  in  which  there  is  an  excessive  amount  of  yolk,  with  the 
nucleus  and  a  small  mass  of  protoplasm  occupying  a  disc  at  the  active 
pole.  This  disc  alone  segments,  and  the  embryo  lies  upon  the  yolk. 
(Illustrated  in  the  eggs  of  fishes,  birds  and  reptiles.)  Fig.  13,  C. 
II.  Peripheral:  in  which  an  excess  of  yolk  collects  at  the  centre  of  the  ovum, 
with  the  protoplasm  at  the  periphery.  The  dividing  nuclei  assume  a 
superficial  position  and  surround  the  unsegmented  yolk.  (Illustrated  in 
the  eggs  of  insects  and  other  arthropods.)  Fig.  13,  D. 


ZOOLOGY 


D 


ABC 

PlG.  13.  Cleavage  and  gastrulation  as  affected  by  yolk  (not  drawn  to  scale).  The  vertical 
rows  A.  B,  C,  and  D  represent  different  classes  of  ova.  A,  an  ovum  with  little  yolk;  B,  one  with 
considerable  yolk  collected  at  the  lower  pole  (P.p.fc  C,  one  with  a  large  amount  of  dense  yolk  crowd- 
ing the  protoplasm  to  one  side  (a.£.) ;  D,  ovum  with  dense  yolk  collected  at  centre.  The  numerals 
(1-4)  indicate  stages  in  cleavage  and  gastrulation:  i,  ova;  2,  4-8  celled  stages  of  segmentation; 
3,  blastospheres,  blastula  stage;  4.  gastrula  stage,  a,  archenteron;  a.p.,  active  pole;  bl,  blastoderm ; 
bp.,  blastopore;  ec,  ectoderm;  en,  entoderm;  ma.,  macrospheres;  mi,  microspheres;  p.p.,  passive 
pole;  s.c.,  segmentation  cavity;  y,  yolk;  y.c.,  yolk  nuclei. 

Questions  on  the  figures. — What  constitutes  the  difference  between  the  active 
and  the  passive  pole?  Judging  from  the  drawings  and  from  your  references  to 
texts  does  gravity  have  any  influence  in  determining  the  position  of  these?  Your 
evidences?  Which  pole  gives  rise  to  ectoderm?  Why  does  the  food  substance 
interfere  with  segmentation?  What  is  the  difference  between  the  segmentation 
cavity  and  the  archenteron?  How  does  the  presence  of  food  substance  modify 
the  formation  of  an  archenteron? 


FROM   SIMPLE   CELL  TO   COMPLEX  ANIMAL  39 

54.  Blastula  and  Morula. — As  cleavage  continues  the  blasto- 
meres  remain  associated  in  a  spherical  mass.     The  individual 
cells  project  beyond  the  general  surface  not  unlike  the  lobes 
of  a  mulberry,  and  for  this  reason  this  stage  is  called  the  morula 
or  mulberry  stage  (Fig.  13,  2).     By  the  growth  of  the  cells  and 
by  the  imbibition  of  water  the  morula  may  become  a  hollow  sphere 
of  cells  (blastula)  the  central  cavity  of  which  is  filled  with  fluid. 
The  cavity  is  termed  the  segmentation  cavity  (Fig.    13,   s.c.). 
Some  animals,  such  as  Volvox,  never  develop  beyond  the  blastula 
stage. 

55.  Gastrula. — In  those  eggs  in  which  the  segmentation  is 
total,  a  next  important  step  is  the  pushing  in  of  that  side  of 
the  blastula  which  corresponds  to  the  original  nutritive  pole. 
The  process  is  known  as  imagination,  and  the  product  as  a 
gastrula  (Fig.  13,  4).     It  takes  place  much  as  one  might  suppose 
one  side  of  a  hollow  rubber  ball  to  be  dimpled  or  infolded  by  the 
exhaustion  of  the  air  within.     The  gastrula  is  to  be  described 
as  made  up  essentially  of  two  layers  of  cells,  one  external  and 
called  ectoderm,  and  one  within  called  entoderm  (Fig.   13,  4). 
The  segmentation  cavity  may  be  wholly  obliterated;  in  that  case 
the  entoderm  and  ectoderm  come  to  lie  in  contact.     The  cavity 
of  the  invagination  of  the  gastrula  is  the  archenteron  or  embryonic 
digestive  tract;  the  opening  into  it,  that  is,  the  mouth  of  the 
gastrula,  is  the  blastopore  (Fig.  13,  bp).     Such  an  organism  as 
Hydra  (see  Fig.  81)  may  be  looked  upon  as  a  permanent  gastrula, 
somewhat  modified  in  form.     In  morulas  in  which  the  seg- 
mentation cavity  is  small  and  the  cells  at  the  nutritive  pole  are 
large  (Fig.  13,  C,  4)  this  simple  condition  is  much  obscured, 
and  invagination  as  described  above  becomes  impossible.     Nev- 
ertheless early  in  development  the  cells  which  produce  the  two 
primitive  layers  are  to  be  distinguished,  and  their  relations  are 
always  substantially  as  detailed.     If  the  term  gastrula  is  applied 
to  these  we  have  to  say  that  they  are  formed  in  some  other  way 
than  by  ordinary  invagination. 

56.  Library  Reference. — Let  students  report  briefly  on  gastrulation  by  over- 
growth (epibole),  and  by  delamination.  Compare  the  results  attained  by  the 
various  methods.  Note  what  is  constant  in  the  methods  and  in  the  results. 


ZOOLOGY 


57.  Germinal  Layers. — The  ectoderm  and  entoderm  have 
thus  far  been  mentioned  as  the  primary  germinal  layers  of  cells. 
Some  of  the  Invertebrates  have  only  these  two  layers,  but  in 


PIG.  14.  Modes  of  forming  mesoderm  (diagrams  modified  from  Whitman  andJSelenka). 
A  and  B,  special  mesoblasts  distinguishable  early  in  segmentation  (Annelid):  A,  surface  view  from 
active  pole;  B,  sectional  view  of  same,  ec,  micromeres  destined  to  form  ectoderm;  en.  macromeres 
destined  to  form  entoderm;  m,  primitive  mesoblast  which  produces  the  mesoderm.  C,  amoeboid 
mesodermal  cells  (c)  budding  from  entoderm  into  the  segmentation  cavity  (j.c.),  in  an  Echinoderm. 
a,  archenteron. 


FIG.  15. 


S.C. 


FIG.  15.  Mesoderm  formed  by  pouches  from  entoderm  after  gastrulation.  A  and  B,  early 
and  later  stages  in  formation  of  mesoderm  and  ccelom.  a,  primitive  gut;  bp.,  blastopore;  cat,  body 
cavity,  formed  from  pockets  of  the  archenteron;  ec.,  ectoderm;  en.,  entoderm;  m.,  mesoderm;  m.so, 
body-wall  mesoderm;  m.sp,,  visceral  mesoderm;  s.c.,  segmentation  cavity. 

Questions  on  figures  14  and  15. — Enumerate  the  three  modes  of  mesoderm 
formation  figured  here.  In  which  type  may  the  mesoderm  be  identified  most  early 
in  the  embryonic  development?  By  comparing  with  other  texts  determine  in 
what  groups  of  animals  the  mesoderm  is  formed  as  in  Fig.  15. 

most  cases  a  third  mass  of  cells  comes  to  be  situated  between  the 
ectoderm  and  entoderm,  from  which  important  organs  are  de- 
rived. The  third  or  middle  layer  (mesoderm)  differs  somewhat 
in  its  origin  in  the  different  groups  of  animals.  It  may  originate 


FROM   SIMPLE   CELL   TO   COMPLEX   ANIMAL  41 

(i)  from  the  multiplication  of  a  few  special  cells  which,  before 
invagination,  in  early  cleavage  stages,  become  distinct  from 
those  that  are  to  form  ectoderm  and  entoderm  (Fig.  14,  A  and 
B,  m)\  (2)  by  means  of  isolated,  wandering  cells  budded  from 
the  other  two  layers,  particularly  the  entoderm  (Fig.  14,  C,  c); 
or  (3)  from  entoderm,  in  the  form  of  pouches  or  of  solid  buds 
of  cells  which  arise  from  the  walls  of  the  archenteron  and  extend 
into  the  segmentation  cavity  (Fig.  15,  m).  In  some  instances 
there  may  occur  a  combination  of  these  methods. 

58.  Coelom. — When   the   mesoderm   develops   by   the  last 
mentioned  method,  i.  e.,  by  the  evagination  of  the  wall  of  the 
primitive  gut  (Fig.  15,  m),  we  see  a  pair  of  folds,  or  a  series  of 
pockets,  the  cavities  of  which  are  at  first  continuous  with  the 
archenteron,  but  later  become  separate  from  it  and  entirely 
surrounded  by  the  mesodermic  layers.     The  outer  wall  of  the 
mesodermic  pouches  joins  the  ectoderm  and  forms  a  body  wall, 
and  the  inner  applies  itself  to  the  entodermal  wall  of  the  gut. 
The  space  between  is  the  ccelom  or  body  cavity.     When  the  meso- 
derm arises  as  a  solid  mass,  instead  of  a  pocket,  the  body  cavity 
is  formed  by  the  splitting  of  the  mass  into  an  inner  and  an  outer 
portion.     When  the  ccelom  is  formed  by  several  pockets  the 
cavities  of  these  may  ultimately  coalesce,  forming  a  single  body 
cavity.     Such  a  cavity  is  found  in  all  the  vertebrates  and  in  the 
higher  invertebrates,  although  it  may  become  more  or  less  ob- 
scured and  modified  in  the  adult. 

59.  Differentiation  of  Organs  and  Tissues. — We  have  al- 
ready in  these  three  layers  and  their  foldings  the  fundamental 
outline  of  that  differentiation  which  is  to  give  us  the  complex 
animal  form  found  in  the  adult.     From  these  layers,  singly  or  in 
combination,  all  the  tissues  and  organs  of  the  body  arise.     The 
various  layers  become  locally  thickened,  folded,  or  otherwise 
modified  in  form  by  rapid  cell  division,  thus  producing  the  begin- 
nings of  organs.     At  a  later  date  differentiation  takes  place 
among  the  cells,  and  tissues  arise  (see  next  chapter).     In  general 
each  layer  gives  rise  to  such  structures  as  its  position  and  relation 
to  the  outer  layers  would  suggest.     This  is  especially  noticeable 
in  the  ectoderm  and  entoderm.     The  ectoderm  is  more  closely 


42  ZOOLOGY 

related  to  the  outside  world,  and  from  it  are  produced  the  pro- 
tective and  sensory  structures.  These  include  the  outer  portion 
of  the  skin  and  the  hard  parts  often  associated  with  it,  and  the 
whole  nervous  system  together  with  the  sensitive  portions  of  the 
organs  of  special  sense,  as  the  retina  of  the  eye.  The  entoderm 
is  derived  from  the  cells  which  contain,  or  at  least  are  closely 
related  to,  the  food  originally  stored  in  the  ovum  (Fig.  13),  and 
it  comes  to  lie  in  the  interior  of  the  embryo.  It  furnishes  the 
lining  of  the  adult  digestive  tract  as  well  as  the  essential  parts 
of  the  glands  and  other  outgrowths  arising  from  it.  The  me  so- 
derm  gives  origin  to  the  muscles  and  to  the  supportive  tissues, 
to  the  blood  vessels  and  blood.  Many  of  the  organs  are  made 
up  of  contributions  from  two  or  all  of  these  germinal  layers. 
Students  must  be  referred  to  special  textbooks  on  embryology 
for  a  more  extended  account  of  the  manner  in  which  the  germinal 
layers  give  rise  to  adult  organs. 

60.  Summary  of  the  Life -cycle  and  the  Meaning  of  the 
Steps. — It  is  important  that  the  student  bring  together  the 
essential  points  in  the  cycle  of  events  that  occur  in  the  life 
history  of  an  organism  from  one  generation  to  the  next. 

1.  Fertilization. — If  we  start  with  the  mature  egg  and  sperm 
ready  to  unite,  we  must  recall  (§50)  that  each  of  these  has  lost, 
by  the  reduction  division,  one-half  of  the  natural  number  of 
chromosomes  in  the  species.     There  is  an  increasing  body  of 
evidence  for  the  belief  that  the  chromosomes  bear  the  hereditary 
characters  from  one  cell  to  another  and  hence  from  one  generation 
to  another.     When  the  nucleus  of  the  sperm  unites  with  the 
nucleus  of  the  egg  in  fertilization  one-half  of  the  chromosomes 
in  the  new  nucleus  thus  comes  from  each  of  the  parents.     Fertili- 
zation may  be  said  then  to  do  two  things:  (a)  it  unites  the 
substances  carrying  hereditary  qualities  from  two  parents  (usu- 
ally); and  (6)  starts  the  development  of  the  egg  (Figs,  n,  12). 

2 .  Cleavage,  and  the  Segregation  of  Germ  Cells. — The  fertilized 
egg  soon  begins  to  divide  (cleavage).     The  resulting  cells  may  be 
much  alike  or  may  be  very  different  in  size  and  contents,  from  the 
very  beginning.     (See  Fig.  13.)    Most  of  these  cells  enter  directly 
into  the  making  of  the  adult  body  by  the  differentiation  into 


FROM   SIMPLE   CELL   TO   COMPLEX   ANIMAL  43 

entoderm  and  ectoderm,  and  later  into  nerve  cells,  muscle  cells, 
gland  cells,  and  the  like.  It  has  been  clearly  shown,  however, 
for  some  species,  that  certain  undifferentiated  cells  are  sooner  or 
later  put  aside,  which  do  not  take  direct  part  in  the  development 
of  the  tissues  of  the  body.  By  tracing  the  history  of  these  cells 
through  the  various  stages  of  embryonic  development  it  is  found 
that  these  are  primordial  or  ancestral  germ  cells  and  that  they 
ultimately  produce  the  sperm  and  egg  cells  with  which  we  are 
familiar.  (See  Figs,  n  and  12.)  It  is  probable  that  something 
similar  will  be  found  to  be  true  of  animals  generally. 

3 .  The  Parallel  Development  of  the  Body  and  the  Germ  Cells. — 
From  this  time  on  two  quite  different  things  are  happening  in 
every  normal  body  as  it  develops:  (a)  the  body  cells  are  multi- 
plying, growing,  and  differentiating  to  form  the  tissues  and  or- 
gans of  the  animal;  and  (6)  the  primordial  germ  cells,  which 
have  come  from  the  same  egg,  and  are  cousins — not  descendants 
of  the  body  cells — are  dividing  and  growing  but  not  differentiating. 
During  whatever  time  is  necessary  for  the  organism  to  develop 
its  body  tissues  and  organs,  the  primordial  germ  cells  are  rinding 
the  permanent  place  they  are  to  occupy  in  the  body  and  are, 
in  cooperation  with  certain  of  the  body  cells,  forming  sex  glands, 
— testes  and  ovaries. 

4.  Sexual  Maturity  and  the  Perfection  of  the  Ova  and  Sperm. — 
In  this  parallel  development  of  the  germ  cells  and  the  body  cells 
it  is  certain  that  they  modify  each  other  profoundly.     The 
germ  cells  depend  for  their  supply  of  food  and  oxygen,  etc.,  upon 
the  activities  of  the  body  cells.     Undoubtedly  the  wastes  of  the 
body  cells  also  influence  the  germ  cells.     However,  it  seems  that 
the  presence  and  products  of  the  germ  cells  even  more  modify 
the  growth  of  the  body.     This  is  especially  true  as  the  germ  cells 
and  glands  approach  maturity.     In  a  number  of  species  it  has 
been  shown  that  the  development  of  certain  parts  of  the  body 
is  very  much  changed  if  the  maturing  sex  glands  are  removed 
early  in  life.     In  general  it  is  believed  that  the  differences  in  the 
bodies  and  the  instincts  of  the  male  and  female  of  a  species  are 
largely  due  to  the  effect  of  the  development  of  sperm  and  ova 
within  the  bodies. 

In  the  last  steps  whereby  the  spermatogonia  produce  sperm 


44  ZOOLOGY 

and  the  oogonia  produce  eggs,  there  are  two  important  happen- 
ings: (a)  the  union  of  the  simple  chromosomes  of  a  nucleus, 
originally  derived  from  the  two  parents,  to  form  the  bivalent 
chromosomes;  and  (b)  the  separation  of  these  at  the  next  division 
in  such  a  way  as  to  reduce  the  simple  chromosomes  to  one-half 
the  number  usually  found  in  the  cells  of  the  species  (Figs.  12,  13). 

It  will  be  recalled  that  the  chromosomes  found  in  any  primor- 
dial germ  or  body  cell  are  descendants  of  chromosomes  that  came 
in  equal  numbers  from  the  egg  and  sperm  nuclei  when  they  first 
united.  We  have  seen  that  every  cell  gets  its  part  of  every  one  of 
these  chromosomes.  It  is  believed,  when  these  chromosomes 
unite  in  pairs  in  the  act  of  maturing,  that  they  do  not  unite  in  a 
chance  manner,  but  that  those  pairing  are  corresponding  chromo- 
somes coming  originally  from  the  mother  and  father.  In  other 
words  this  union  is  the  final  scene  in  the  mixing  of  male  and  female 
qualities  which  began  when  the  nuclei  united  in  fertilization. 
The  protoplast  of  ovum  and  sperm  unite  at  fertilization;  but 
the  final  union  of  the  chromosomes  of  the  ovum  and  sperm  is 
delayed  until  near  the  time  of  the  next  union  of  sperm  and  ova. 
It  is  possible  that  these  bivalent  chromosomes  may  be  exchang- 
ing material  before  their  final  separation. 

The  reduction  of  the  chromosomes  to  one-half  their  typical 
number  is  clearly  a  device  which  maintains  the  standard  number 
when  the  ovum  and  sperm  unite.  If  they  were  not  reduced  the 
amount  of  chromatin,  and  the  accompanying  hereditary  effects, 
would  be  doubled  at  every  fertilization. 

61.  Summary. 

1.  All  the  higher  animals  begin  life  as  a  single  cell  and  reach 
their  adult  condition  by  a  continuous  series  of  divisions.     By 
the  growth  and  specialization  of  the  cells .  arising  from  these 
divisions  the  great  complexity  of  the  adult  body  is  produced. 

2.  This    initial    cell — the   fertilized    ovum — represents    the 
fusion  of  two  independent  and  unlike  cells:  the  ovum  (female) 
and  the  spermatozoon  (male). 

3.  Before  the  union  (fertilization)  occurs,  the  ovum  reduces 
its  nuclear  material,  by  two  successive  divisions,  to  one-fourth 
its  original  amount   and  the  chromosomes  to  one-half  their 


FROM   SIMPLE   CELL   TO   COMPLEX   ANIMAL  45 

original  number,  without  a  corresponding  reduction  of  the  cyto- 
plasm. The  spermatozoon  in  its  development  undergoes  a 
similar  reduction  of  chromosomes. 

4.  After  the  union  of  the  male  and  female  cells  the  fertilized 
ovum  divides  rapidly  (segmentation  or  cleavage)  forming  a  mass 
of  cohering  cells.     The  nature  of  these  cells  and  of  the  mass 
depends  much  on  the  amount  of  yolk  in  the  ovum  and  on  its 
distribution. 

5.  By  processes  which  differ  in  different  animals  according 
to  the  nature  of  the  segmentation,  the  cells  become  arranged 
with  a  layer  outside  (ectoderm),  a  layer  within  (entoderm),  and 
from  these  a  third  layer  or  mass  of  cells  lying  between  the  other 
two  (mesoderm).     The  entoderm  bounds  a  cavity  (archenteron) 
which  communicates  by  a  pore  (blastopore)  with  the  outside 
world.     Within  the  mesoderm  may  be  found  a  cavity  (coelom). 

6.  The  ectoderm  gives  rise  to  the  outer  portions  of  the  skin, 
its  protective  and  sensory  structures,  to  the  nervous  system, 
and  frequently  to  the  lining  of  the  openings  into  the  body.     The 
entoderm  lines  the  principal  part  of  the  digestive  tract.     The 
mesoderm  gives  rise  to  most  of  the  other  structures  of  the  body. 

62.  Suggestive  Topics  for  Library  Work. 

1.  What  suggestions  have  been  offered  as  to  the  advantage 
of  the  addition  of  the  male  nucleus  to  that  of  the  female  in 
fertilization?     Has   a  similar  result  ever  been   attained  arti- 
ficially by  means  of  chemical  or  other  stimuli  ? 

2.  What  explanations  have  been  offered  as  to  the  signifi- 
cance of  the  process  of  maturation?     Trace  the  maturation 
of  the  sperm  cells  more  fully. 

3.  What    classification    of    ova    do    the  text-books    make? 
What  is  the  basis  of  the  classification?     To  what  extent  do 
eggs  of  different  animals  vary  in  size,  shape,  envelopes,  etc.  ? 
Give  examples. 

4.  Is  there  any  explanation  of  the  fact  that  there  is  such 
a  difference  in  the  amount  of  food  substance  in  the  eggs  of 
different  animals  ? 

5.  Trace  out  by  reference  to  a  text-book  of  embryology  the 
principal  changes  by  which  the  adult  digestive  tract  is  derived 


46  ZOOLOGY 

from  the  simple  condition  found  in  the  gastrula  (archenet- 
ron).  What  is  the  fate  of  the  blastopore?  How  does  the 
permanent  mouth  originate? 

63.  Exercises  for  the  Laboratory. 

The  teacher  should  secure  demonstrations  of  some  of  the 
smaller  ova  (as  of  the  snail,  fish,  sea-urchin)  for  examina- 
tion with  the  microscope.  Compare  the  ovum  taken  from  the 
ovary  of  a  hen  with  a  new  laid  egg,  noting  especially  the  struc- 
ture of  the  latter.  Obtain  spermatozoa  from  the  testis  of  a 
recently  killed  animal  (as  mouse,  fowl,  etc.),  and  examine  with 
highest  powers  of  the  microscope.  If  possible  secure  permanent 
mounts  of  segmenting  eggs  of  sea-urchins,  showing  the  2-,  4- 
8-celled  stages. 


CHAPTER  V 

CELLULAR  DIFFERENTIATION.— TISSUES 

64.  Two  things  of  importance  happen  to  the  body  as  the 
organism  develops  from  the  simple  condition  of  the  ovum  to  the 
great  complexity  of  structure  in  the  adult :  (i)  the  increase  in  the 
number  of  cells,  which  is  quantitative  in  nature,  and  (2)  the 
differentiation  of  cells,  whereby  the  cells  of  the  various  parts 
become  very  diverse  in  shape,  composition,  and  powers.     This 
is  a  qualitative  change.     It  is  not  yet  fully  known  how  much 
of  the  difference  in  the  cells  of  the  various  tissues  is  due  to 
qualitative  differences  in  the  daughter  cells  of  a  given  division, 
and  how  much  is  due  to  external  influences  and  the  interrela- 
tions of  the  cells  after  division.     We  know,  for  example,  that 
gravity  acting  on  the  food  substance  of  the  ovum  before  division 
does  produce  such  differences  among  the  daughter  cells  of  the 
early  cleavage  stages  as  lead  to  results  as  diverse  as  ectoderm 
and  entoderm.     Doubtless  there  are  also  internal  processes  that 
tend  to  give  rise  to  similar  differences.     On  the  other  hand,  it  has 
been  shown  by  experiment  that,  even  as  high  up  in  the  animal 
scale  as  the  lowest  vertebrates,  the  blastomeres  of  the  two-  or  four- 
celled  stage  may  be  shaken  apart  and  each  develop  into  a  small 
but  perfect  embryo.     This  experiment  shows  that  up  to  this 
stage  no  specialization  has  taken  place  which  limits  the  products 
that  come  from  these  cells.     The  blastomeres  do  not  so  develop 
after  the  8-  or  i6-celled  stage  is  reached,  so  far  as  is  known. 
We  are  ignorant  of  the  causes  which  determine  that  one  cell 
shall  develop  into  a  muscle  cell  and  its  neighbor  into  a  bone 
cell. 

65.  Tissues. — A  tissue  is  to  be  defined  as  a  group  of  similar 
cells  suited  by  their  differentiation  to  the  performance  of  a 
definite  function.     This  differentiation  affects  the  size,  shape, 
and  the  interrelations  of  cells,  and  likewise  the  chemical  and 

47 


48  ZOOLOGY 

physical  structure  of  the  protoplasm,  in  such  a  manner  as  to 
cause  great  variation  in  their  powers  and  activities.  The 
chemical  differences  are  especially  shown  in  excretion  and  se- 
cretion, whereby  various  sorts  of  materials  are  deposited  within 
and  between  the  cells  of  the  different  tissues.  The  material 
deposited  between  the  cells  is  known  as  intercellular  substance. 
The  intercellular  substance  differs  much  in  character  and 
amount.  Both  the  cells  and  the  intercellular  substance  are 
important  in  enabling  the  tissue  to  perform  its  work.  In  gen- 
eral if  the  tissue  is  active  (as  muscle)  the  cellular  differentia- 
tion is  the  important  point;  if,  however,  the  function  is  a  more 
passive  one,  as  support  or  protection,  the  nature  of  the  inter- 
cellular substance  rather  than  the  cells  determines  its  character 
(bone,  connective  tissue). 

66.  Classification'  of  Tissues. — From  a  physiological  point 
of  view  tissues  may  be  classed  in  one  of  two  groups:  vegeta- 
tive, and  active.     The  vegetative  tissues  are  those  which  per- 
form the  more  passive  functions,  as  nutrition,  protection,  sup- 
port, etc.     They  resemble  the  plant  tissues  in  their  functions. 
The  two  chief  classes  of  vegetative  tissues  are:  epithelial  or 
bounding   tissues,    and   supportive  or   connective  tissues.     The 
active  tissues  may  be  looked  upon  as  the  characteristic  tissues 
of  animals.     The  muscular  and  nervous  tissues  belong  to  this 
group. 

67.  Epithelial  Tissues. — This  tissue  is  characterized  by  its 
primitive  form,  i.e.,   by  its  relative  lack  of  differentiation,  by 
the  fact  that  it  is  the  first  to  appear  in  individual  development 
(ectoderm  and  entoderm  in  the  gastrula),  and  by  the  absence 
of  intercellular  substance.     It  is  a  bounding  tissue  and  consists 
typically  of  a  single  layer  of  cells,  although  several  layers  may 
occur.     Epithelium  bounds,  by  its  own  cells  or  their  products, 
the  outside  of  the  body,  the  lumen  of  the  digestive  tract  and 
its  outgrowths,  as  well  as  the  body  cavity  and  the  structures 
cpntained  in  it. 

68.  Kinds   of    Epithelial    Tissue. — Located    in    a   position 
superficial  to  the  other  tissues,  epithelium  is  subject  to  a  wide 
range  of  variation  both  as  to  form  and  function.     Besides  its 


CELLULAR   DIFFERENTIATION 


49 


primary  work  as  a  protective  layer,  the  epithelium  may  have 
a  glandular  function,   being  favorably  situated  for  the  final 


LA. 


FIG.  16.  Various  kinds  of  epithelial  cells  (semi-diagrammatic).  A,  columnar;  B,  cuboidal; 
C,  pavement;  D  and  E,  ciliate  (sectional  views).  In  F  is  shown  the  surface  view  of  pavement 
epithelium.  cl.t  cilia;  cu.,  cuticula. 

Questions  on  the  figures. — For  what  different  uses  would  you  judge  these 
variously  shaped  epithelial  cells  to  be  suited?  Under  what  circumstances  and  on 
what  surfaces  would  you  expect  to  find  each  type?  Compare  with  your  reference 
texts  and  see  if  they  are  so  found.  Under  what  circumstances  is  a  cuticula  to  be 
expected?  Where  would  it  be  a  disadvantage?  What  are  cilia? 

-  <  FIG.  17. 

,'OL 


PIG.  17.     Glandular  epithelium,     a,  goblet  or  slime  cells, — unicellular  glands;  6,  similar  cells 
which  have  become  depressed  below  the  surface,  and  empty  their  secretion  through  a  duct. 

Questions  on  the  figures. — Are  the  glandular  cells  modified  epithelial  cells?     In 
what  respects  do  they  differ  from  the  cells  about  them  ? 

elimination  of  products  from  the  body.  Since  it  is  especially 
exposed  it  is  the  layer  best  adapted  by  position  to  receive  those 
external  stimuli  which  we  know  to  play  such  an  important  r61e 


50  ZOOLOGY 

in  the  life  of  all  organisms.  The  position  of  the  epithelium 
also  renders  it  specially  liable  to  destruction.  To  compensate 
for  this  its  primitive  or  undifferentiated  character  makes  it 
particularly  capable  of  regenerating  portions  of  itself  which 
may  have  been  lost.  Epithelium  is  often  especially  active  also 
in  the  regeneration  of  other  than  simple  epithelial  structures,  as, 
for  example,  the  regeneration  of  nervous  cells  in  a  cut  earth- 
worm. In  close  connection  with  this  latter  regenerative  quality 

FIG.  18. 


FIG.  18.  A.  series  of  diagrams  showing  progressive  stages  in  the  development  of  a  multicellular 
gland  from  an  area  of  glandular  epithelial  cells.  C  and  D  show  two  somewhat  different  types  of 
gland, — the  cup-shaped  and  the  tubular,  e.  bounding  epithelium;  g,  gland  cells;  d,  duct;  ct  connect- 
ive tissue. 

Questions  on  the  figures. — How  do  the  compound  glands  seem  to  arise  from 
the  simpler  condition?  What  is  the  evidence  that  glands  are  lined  throughout 
with  epithelium?  What  is  gained  in  the  sinking  of  the  glands  below  the  surface? 


is  to  be  considered  the  fact  that  the  reproductive  or  sexual  cells, 
by  which  new  individuals  are  produced  arise  from  an  epithelium. 
The  foregoing  enumeration  of  functions  suggests  the  physiolog- 
ical classification  of  epithelial — bounding,  glandular,  sensory, 
and  reproductive  epithelia.  The  same  layer  may  fulfill  several 
of  these  functions  at  once. 


CELLULAR   DIFFERENTIATION  51 

69.  Bounding  Epithelium. — The  ordinary  protective  epithelium  may  be  made 
up  of  cells  cuboidal  in  shape  (Fig.  16,  B),  or  columnar  (Fig.  16,  A),  or  much 
flattened  (Fig.  16,  Q.  In  extreme  cases  of  flattening  and  hardening  we  have 
squamous  epithelium,  e.g.,  the  outer  cells  of  the  human  epidermis.  Motile  proto- 


FIG.  19.  Sensory  and  muscular  epithelium.  A,  sensory  epithelium,  from  Worm,  showing 
some  of  the  epithelial  cells  (e)  modified  into  sensory  cells  (s).  B,  epithelial  cells  from  Hydra  showing 
contractile  or  muscular  processes  at  base  (m). 

Questions  on  the  figures. — Is  there  anything  to  suggest  that  the  sensory  cells 
are  modified  epithelial  cells?  What  are  the  principal  changes  which  they  have 
undergone  as  compared  with  the  unmodified  epithelium? 

FIG.  20. 


FIG.  20.  Diagram  of  a  portion  of  the  ovary  of  Sea-urchin  showing  the  eggs  arising  from  the 
epithelium  (reproductive  epithelium)  by  constriction,  e,  epithelium;  <?,  ova  in  different  stages  of 
growth. 

Questions  on  the  figure. — What  is  an  ovary  in  its  simplest  form?  Is  the  re- 
productive epithelium  ectoderm al,  entodermal,  or  mesodermal  in  origin,  as  a 
rule? 


plasmic  projections  often  extend  from  the  free  surface  of  the  epithelium.  Flagel- 
late epithelium  (Fig.  79,  D)  has  one  such  projection  from  each  cell,  whereas  ciliate 
epithelium  (Fig.  16  D,  E)  has  numerous  small  ones.  Cilia  are  more  common  in 


52 


ZOOLOGY 


the  lower  groups  of  animals,  but  are  found  even  in  mammals,  in  the  moist  internal 
passages,  as  in  the  nose,  trachea,  etc. 

Membranes  bounding  the  body  cavity  are  called  serous  membranes  (endothe- 
lium).  The  lining  of  the  digestive  tract  is  described  as  a  mucous  membrane. 

Epithelial  cells  often  secrete  upon  their  outer  surface  a  layer  of  material 
(cuticula},  which  serves  to  protect  the  cells  beneath  and  the  organism  as  a  whole 
from  external  influences  (as  the  covering  of  the  cray-fish).  From  the  epithelium 
arise  various  outgrowths,  as  scales,  hair,  feathers,  and  the  like. 

70.  Glandular  Epithelium. — The  ordinary  columnar  or  pavement  epithelium 
may  here  and  there  present  cells  or  areas  of  cells  which  are  specially  active  in 
producing  and  pouring  out  on  their  free  surface  certain  materials,  called  secretions. 
In  its  simplest  form  the  gland  or  secreting  surface  may  consist  of  a  single  cell,  as 
the  goblet  or  slime  cells  (Fig.  17,  a). 

Such  a  cell  may  become  much  enlarged  and  sink  below  the  general  level  of  the 
epithelium,  retaining  in  the  meantime  a  narrow  connection  with  the  exterior  (Fig. 
17,  &).  Multicellular  glands  represent  areas  of  such  cells  which  have  sunk  below 


FIG.  21. 


FIG.  21.  Section  through  ovary  of  a  young  Mammal  (modified  from  Wiedersheim) .  The  eggs 
(o)  are  seen  to  be  formed  from  the  epithelium  by  a  process  somewhat  more  complex  than  in  Fig.  18. 
c,  connective  tissue  of  ovary;  e,  epithelium;/,  follicle  of  epithelial  cells  in  which  the  ova  ripen;  o,  ova 
in  different  stages  of  ripeness. 

Questions  on  the  figure. — In  the  ovary  of  the  mammal  what  additional  service 
does  the  epithelial  layer  render  the  ovum  after  its  formation  ?  Is  it  apparent  that 
there  is  anything  gained  by  the  sinking  of  the  ovarian  follicles  into  the  tissue  of  the 
ovary,  instead  of  escaping  immediately,  as  in  Fig.  20  ? 

the  surrounding  surface,  forming  a  tube-  or  flask-shaped  cavity,  which  may  become 
very  much  branched.  Glands  with  such  branched  ducts  are  described  as  compound. 
They  consist  of  numerous  final  secretory  sacs  communicating  by  ductules  with  a 
common  duct  or  outlet  to  the  surface.  Transitional  conditions  between  the 
simple  secretory  epithelium  and  the  compound  gland  may  be  seen  in  Fig.  18. 

71.  Sensory  Epithelium. — In  the  lower  animals  there  may  be  found  here  and 
there  over  the  surface  of  the  body  modified  epithelial  cells,  which  are  specially 


CELLULAR   DIFFERENTIATION  53 

capable  of  being  stimulated  by  contact  or  other  stimuli  to  which  the  organism  may 
be  exposed.  Likewise  in  higher  forms  we  find  highly  specialized  areas  of  sensitive 
cells,  which  can  be  shown  to  belong  primarily  to  the  epithelium.  These  are  the 
end  organs  of  special  sense,  as  touch,  sight,  and  the  like,  and  they  get  their  special 
value  from  their  connection  with  what  will  be  described  presently  as  the  nervous 
tissues  of  the  central  nervous  system.  The  sensory  cells  are  typically  spindle-like 
or  even  hair-like  in  form,  often  extended  as  fine  fibres  at  the  inner  end,  whereby 
connection  is  established  with  the  nerves  (Fig.  19,  A). 

72.  Reproductive  Epithelium. — The  sexual  cells,  both  male  and  female,  arise 
from  epithelium,  ectodermal,  entodermal,  or,  as  is  usually  the  case,  mesodermal. 
The  budding  of  the  sexual  epithelium,  in  the  development  of  the  germ  cells,  sug- 
gests the  formation  of  glands  (Figs.  20,  21).  The  sexual  cells  often  develop  at  the 
expense  of  the  epithelial  and  other  cells  about  them. 

73.  Supportive  or  Connective  Tissues. — This  class  of  tissues 
embraces  the  bulk  of  the  non-active  tissues  in  animals.  They 
vary  much  in  appearance  and  structure,  agreeing  in  little  except 
in  their  mesodermic  origin,  their  passivity,  and  in  the  preva- 

FlG.    22. 


FIG.  22.     Cellular   Connective   Tissue,   showing  large   vacuoles,   v,   in   the    protoplasm. 

Questions  on  the  figure. — Would  you  say  that  these  cells  are  of  a  high  or  a  low 
order  of  differentiation?  Why?  Is  there  any  intercellular  substance?  Where  is 
tissue  of  this  kind  found?  (See  reference  texts.) 

lence  of  intercellular  substance.  The  intercellular  substance 
gives  the  distinctive  character  to  the  connective  tissues,  the 
cells  having  a  relatively  unimportant  place  after  the  produc- 
tion of  the  intercellular  substance.  The  general  function 
of  the  supportive  tissues  is  to  bind  and  sustain  the  more  active 
tissues  in  their  relations  to  the  body  as  a  whole.  The  classifica- 
tion of  supportive  tissues  is  based  on  differences  in  the  intercellu- 
lar substance.  This  may  be  fluid  (as  in  blood)  or  solid  (as  in 
bone) ;  it  may  be  homogeneous  (as  in  some  forms  of  cartilage) ,  or 


54 


ZOOLOGY 


fibrous;  it  may  be  almost  wholly  organic,  or  very  largely  inor- 
ganic. The  principal  classes  are  cellular  connective  tissues,  ge- 
latinous connective  tissue,  fibrous  connective  tissue,  cartilaginous 
tissue,  and  osseous  tissue. 

FIG.  23. 


FIG.  23- 


Gelatinous  connective  tissue,  showing  stellate  cells  (c),  epithelium  («),  the  gelatinous 
intercellular  substance  (s),  and  the  intercellular  fibres  (/). 


Questions  on  the  figure. — What  seems  to  be  the  relation  of  the  epithelial  layer 
to  the  tissue  below  it?  What  classes  of  cells  are  found  in  the  gelatinous  tissue? 
What  is  their  origin?  What  is  the  nature  of  the  intercellular  substance?  Are  the 
fibres  cellular  or  intercellular? 

74.  Cellular  or  Vesicular  Tissue  forms  an  exception  to  the  general  rule  of  abun- 
dant intercellular  substance.     It  is  an  embryonic  tissue, — a  forerunner  of  the  more 
permanent  tissues, — and  is  chiefly  interesting  from  that  fact.     The  cells  have 
large  vacuoles  or  vesicles  which  are  enveloped  by  a  thin  layer  of  protoplasm  (Fig 
22).     It  is  found  in  the  notochord  of  vertebrates. 

75.  Gelatinous  tissue  has  a  matrix  of  intercellular  substance  enveloping  stellate 
cells,  the  radiating  projections  of  which  serve  to  connect  them.     Fibres  are  often 
developed  in  the  matrix.     This  tissue  is  abundantly  found  in  the  jelly-fish  (see 
Fig.  23). 

76.  Fibrous  connective  tissue  has  in  its  ground  substance  a  rich  supply  of 
fibrils   variously   arranged.     The   cells   or  corpuscles  are   often   elongated   and 
branched.     If  the  intercellular  fibres  cross,  running  in  various  directions,  a  loose 
yielding  tissue  results,  as  in  the  ordinary  connective  tissue  about  the  muscles  and 
nerves  (Fig.  24,  A);  if  the  fibres  are  parallel  the  tissue  naturally  becomes  more 
compact.     There  are  two  types  of  the  more  compact  sort  differing  in  the  quality 
of  the  fibres.     The  latter  may  be  white  and  inelastic,  as  in  tendons,  or  yellow  and 
elastic.     Fat  is  frequently  deposited  as  spherical  drops  of  oil  (Fig.  24,  B)  in  the  cells 
of  connective  tissue. 

77.  Cartilage. — In  cartilage  the  intercellular  matrix  is  much  firmer  than  in 
those  tissues  already  described.     It  may  appear  homogeneous  as  in  rib  cartilage 


CELLULAR  DIFFERENTIATION 


55 


(Fig.  25,  A) ;  or  it  may  contain  numerous  fibres  which  give  coherence  and  elasticity. 
The  cells  are  usually  rounded  except  where  they  have  been  flattened  by  mutual 

FIG.  24. 


FIG.  24.  Fibrous  connective  tissues.  A,  ordinary  connective  tissue  found  binding  muscle 
and  nerve  fibres,  showing  the  fibrous  intercellular  substance.  The  cells  (c)  are  never  conspicuous 
in  this  tissue.  B,  adipose  connective  tissue  showing  fat-laden  cells  among  the  fibres  (/).  o,  oil 
droplets  in  the  cells. 

Questions  on  the  figures. — In  these  two  types  of  tissue  which  element  gives 
special  character  to  the  tissue,  the  cells  or  the  intercellular  substance  ?  How  would 
the  deposition  of  large  drops  of  oil  in  the  cell  affect  the  activity  of  the  cell?  Why? 
Why  are  fatty  deposits  less  hurtful  amid  connective  tissue  than  elsewhere  in  the 
body? 

FIG.  25. 


FIG.  25.  Cartilage.  A,  Hyaline  cartilage;  B,  fibrous  cartilage.  In  the  latter  a  large  portion 
of  the  intercellular  substance  is  conspicuously  fibrous.  The  cells  occur  in  pockets  (p)  in  the  matrix;, 
f,  intercellular  fibres. 

Questions  on  the  figures. — What  are  the  points  of  similarity  and  of  difference 
in  the  two  types  of  cartilage?  In  what  manner  do  the  multicellular  pockets  arise? 
What  is  the  nature  and  origin  of  the  intercellular  substance  in  each  case? 

pressure,  and  usually  occur  in  pockets  in  the  matrix.  Cartilage  is  bounded  on  its 
free  surfaces  by  a  fibrous  membrane,  the  perichondrium.  This  membrane  assists  in 
the  growth  of  the  cartilage.  There  are  no  blood  capillaries  in  cartilage. 


ZOOLOGY 


Salts  of  lime  may  be  deposited  in  the  intercellular  substance,  giving  it  some  of 
the  qualities  of  bone. 

78.  Osseous  or  Bony  Tissue. — These  tissues  are  found  only  in  vertebrates,  and 
are  the  most  complicated  of  the  supportive  tissues.  The  firm  matrix  which  is 
secreted  by  the  bone  cells  consists  of  a  mixture  of  organic  substance  and  inorganic 
matter,  especially  the  salts  of  lime.  The  cells  with  their  fine  filamentous  branches 
occur  more  or  less  regularly  between  thin  plates  or  lamella  of  the  bony  material. 
A  cross-section  of  one  of  the  long  bones  shows  the  typical  condition.  The  perios- 
teum is  a  superficial  fibrous  membrane  about  the  bone,  well  supplied  with  blood 
vessels.  Its  inner  layer  of  cells  is  capable  of  producing  bone.  Within  this  is  a 
region  of  firm  bone,  in  which  a  series  of  lamellae  are  parallel  with  the  surface  of 
the  periosteum.  Between  the  lamellae  occur  the  spaces  (lacuna)  occupied  by  the 
bone-cells  which  have  been  left  behind  as  the  matrix  was  deposited.  Deeper  in 

FIG.  26. 


FIG.  26.  Bony  Tissue.  A,  portion  of  cross-section  of  a  bone,  the  upper  portion  of  the  figure 
representing  the  outer  surface  of  the  bone,  just  beneath  the  periosteum.  The  open  spaces,  h,  are 
Haversian  canals;  I,  lacuna,  occupied  in  life  by  bone  cells.  The  minute  canals  through  the  bone 
connecting  the  lacunae  are  canaliculi.  B,  a  portion  of  one  Haversian  system  much  magnified,  h, 
Haversian  canal,  containing  artery  (a),  vein  (n),  lymphatic  spaces,  nutritive  cells;  c,  canaliculi;  I, 
lacurse;  la,  plate  of  bony  intercellular  substance. 

Questions  on  the  figures. — How  does  bone  compare  in  appearance  and  structure 
with  the  other  supportive  tissues?  What  is  the  really  living  part  of  bone?  How 
is  its  intercellular  substance  laid  down?  How  are  the  cells  in  the  bone  nourished? 
How  do  they  come  to  lie  in  the  solid  bone?  What  changes  occur  in  this  type  of 
tissue  with  age?  What  is  the  function  of  the  Haversian  canal? 

the  bone  the  lamellae  and  cells  are  in  concentric  layers  about  the  numerous  blood 
vessels  (occupying  spaces  known  as  Haversian  canals)  which  penetrate  the  bone, 
chiefly  in  a  longitudinal  direction.  The  included  bone-cells  communicate  with 
each  other  and  with  the  blood  vessels  by  processes  which  occupy  minute  canals 
(canaliculi)  in  the  intercellular  substance  (Fig.  26).  Within  this  region  and  imme- 
diately surrounding  the  central  cavity  of  the  bone  is  often  a  mass  of  spongy  bone 
in  which  the  regularity  of  arrangement  of  the  cells  is  lost.  Bone  may  be  formed 
by  replacing  cartilage,  or  wholly  independent  of  it. 


CELLULAR   DIFFERENTIATION 


57 


PIG.  27.     Blood  corpuscles  (amphibian),     c,  colored  corpuscles,  flatwise  and  in  profile;  I,  colorless 

corpuscles  (leucocytes). 

FIG.  28. 


FIG.  28. 


Blood  corpuscles  (human),     c,  colored;  I,  leucocytes. 
rows  with  the  sides  in  contact. 


The  red  cells  tend  to  collect  in 


Questions  on  figures  27  and  28.  —  Compare  —  by  means  of  the  figures,  the  text 
and  reference  books  —  the  colored  and  colorless  corpuscles  of  these  two  types  of 
vertebrates  and  note  the  differences.  In  what  other  respects  do  the  colored  cells 
differ  from  the  white?  Which  are  the  less  highly  differentiated?  Reasons  for 
your  view?  Why  are  the  colorless  corpuscles  also  called  phagocytes? 

Dentine  and  enamel,  though  differing  in  structure  from  bone,  are  to  be  looked 
upon  as  belonging  to  the  same  class  of  tissues.  They  differ  chiefly  in  the  fact  that 
no  cellular  elements  are  included  in  the  secretion.  They  are  thus  harder  and  denser 
than  bone. 

79.  We  find  all  stages  of  transition  between  the  more  sim- 
ple and  more  complex  supportive  tissues,  and  it  may  be  seen 
furthermore  that  there  is  a  fundamental  embryological  sequence. 
In  the  development  of  the  organism  the  simpler  connective  tis- 
sues give  place,  by  transformation  or  substitution,  to  the  more 
complex.  The  cellular  connective  tissue  of  early  life  is  replaced, 
for  example,  by  cartilage,  and  this  may  be  transformed  into 
bone  in  adult  life. 


58  ZOOLOGY 

80.  Nutritive  Fluids. — The  body  fluids  known  as  blood  and  lymph  are  frequently 
classed  among  the  supporting  tissues,  the  fluid  portion  being  regarded  as  the  inter- 
cellular substance  and  the  corpuscles  as  the  cells.  They  differ  however  from  the 
ordinary  tissues  in  the  important  fact  that  the  intercellular  substance  is  not  pro- 
duced by  the  cells.  In  the  vertebrates  these  cells  are  of  two  kinds,  the  amaboid 
or  colorless  and  the  colored.  Both  kinds  occur  in  the  blood;  the  colorless  alone  are 
found  in  the  lymph.  The  colored  corpuscles  are  relatively  numerous  and  are  disc 
shaped.  Regarded  as  cells  they  present  a  series  of  degenerative  changes  which 
results  in  a  loss  of  the  distinctively  protoplasmic  character,  by  the  substitution 
of  certain  proteid  substances,  one  of  which — haemoglobin — is  notable  for  its  affinity 
for  oxygen.  The  degeneracy  may  go  to  the  extent  of  the  entire  loss  of  the  nucleus, 
as  in  the  mammals.  The  colorless  cells  have  the  power  of  independent  motion 
such  as  is  found  in  the  amoeba,  and  may  ingest  solid  particles  of  food.  The  body- 
fluids  of  the  invertebrates  contain  as  a  rule  only  colorless  corpuscles,  and  are  there- 
fore more  like  the  lymph  of  the  vertebrates.  When  their  blood  is  colored  it  is 
usually  from  pigment  in  the  plasma  or  fluid  portion  of  the  blood.  In  addition  to 
the  cells  the  blood  carries  a  rich  supply  of  proteid  and  other  substances  for  use  in 
the  tissues,  and  of  waste  products  in  process  of  removal  from  the  body. 

81.  Muscular  Tissue. — The  remaining  tissues  are  charac- 
teristically active.  Muscular  tissue  by  its  contractility  has  the 
power  of  producing  movements  of  the  parts  to  which  it  is  at- 
tached. This  contractility  of  muscle  may  be  looked  upon  as  a 
specialization,  and  a  limitation  in  direction,  of  the  power  of 
contraction  which  we  have  seen  to  be  resident  in  all  living  proto- 
plasm. Muscular  tissue  differs  somewhat  in  structure  and 
degree  of  differentiation  in  various  animals,  but  in  general  agrees 
in  the  presence  of  elongated  fibres  which  are  to  be  considered  as 
modified  cells  or  parts  of  cells.  The  contractile  muscle  substance 
is,  in  part  at  least,  a  plasmic  product  rather  than  mere  proto- 
plasm; yet  it  differs  from  the  intercellular  substance  of  the  tis- 
sues already  described  in  that  it  is  deposited  within  rather  than 
among  the  cells. 

Two  stages  in  the  differentiation  of  muscular  substance  are 
to  be  noted:  (i)  the  fibres  may  be  plain,  in  which  case  we  find 
elongated,  contractile  single  cells  without  conspicuous  external 
differentiation  (Fig.  29);  (2)  cross-striate  fibres,  which  always 
show  conspicuous  differentiation  of  parts  in  each  fibre  as  seen 
under  the  microscope.  The  plain  fibres  are  characteristic 
of  sluggish  animals,  and  those  parts  of  animals  whose  muscular 
action  is  least  prompt  in  response  to  the  nervous  stimuli  (e.  g., 
digestive  tract  in  vertebrates) .  The  cross-striated  fibre  usually 


CELLULAR   DIFFERENTIATION 


59 


represents  several  incompletely  separated  cells,  or  a  multi- 
nucleate  condition  of  a  much-grown  and  metamorphosed  single 
cell.  In  both  classes  the  fibres  are  made  up  of  numerous  minute 
strands  or  fibrillcs  which  in  the  plain  muscle  are  homogeneous 
throughout,  but  in  the  cross-striated  are  made  up  of  alternating 
segments  of  lighter  and  darker  optical  appearance  (Fig.  30,  B). 

FIG.  29. 


FIG.  29.     Plain  muscle  fibres,     n,  nucleus  of  muscle  cell;  p,  undifferentiated  cell  piotoplasm;  p' , 
the  differentiated  contractile  portion  of  the  cell. 

Questions  on  the  figure. — What  are  the  two  principal  portions  of  these  cells? 
How  do  very  young  muscle  cells  compare  with  older  ones  in  the  relative  amount 
of  these  portions  in  the  cell?  Which  is  the  more  highly  differentiated  portion? 
Where  are  such  tissues  found  in  the  animal  body  ?  Why  are  muscle  fibres  elongated  ? 

The  undifferentiated  protoplasmic  remnant  is  often  very  small 
in  amount,  and  is  collected  about  the  nucleus  (Figs.  29,  30).  It 
may  be  at  the  surface  of  the  fibre  or  in  the  center,  enveloped  by 
the  contractile  matter.  A  thin  membrane  (sarcolemma)  binds 
the  fibrillae  into  fibres.  The  fibres  are  bound  together  by 
strands  of  connective  tissue  into  bundles,  and  of  these  bundles 
the  muscle  is  made  up. 


6c 


ZOOLOGY 


FIG.  30. 


FIG.  30.  Diagram  of  nervous  and  cross-striate  muscular  tissue,  showing  the  mode  of  connec- 
tion between  nerve  fibres  and  muscle  fibres.  A,  nerve  cell  (g)  connected  with  muscle  fibre  (mf.) 
by  nerve  fibre  («./.).  The  muscle  fibre  (m./.)  is  composed  of  numerous  fibrils  (/)  which  are  made  up 
lengthwise  of  alternating  discs  of  lighter  and  darker  substance.  These  fibrils  are  shown  more 
highly  magnified  in  B  and  C.  In  B  the  fibril  is  uncontracted;  in  C  it  is  contracted.  D,  nerve  fibre 
more  highly  magnified  showing  a,  axis;  m,  medullary  sheath;  and  s,  Schwann's  sheath;  ax.,  axon; 
d,  dendron;  n,  node;  n.m.,  nerve-muscle  plate. 


CELLULAR   DIFFERENTIATION  6 1 

82.  Origin  of  Muscle  Tissue. — In  those  animals  in  which  a  true  mesodermis 
wanting,  the  epithelial  cells  may  develop,  at  their  inner  extremity,  contractile 
roots,  either  plain  or  striate  (Hydra,  Fig.  19,  B).  These  cells  may  wholly  lose 
their  epithelial  quality  and  position  and  become  entirely  muscular.  In  the  higher 
animals  this  is  very  much  modified  by  the  early  appearance  of  separate  mesoderm 
from  which  the  whole  muscular  system  is  derived. 

83.  Nervous  Tissue:  its  Functions. — The  nervous  tissues 
are  in  close  relation  on  the  one  hand  to  the  sensory  epithelium 
and  on  the  other  to  the  muscular  tissue.     Through  the  former 
they  receive  the  stimuli  from  the  outside  world;  by  means  of 
their  connection  with  the  latter  they  are  enabled  to  effect  re- 
sponses.    The  reception  of  stimuli,  the  transmission  of  the  results 
of  stimulation,  and  the  initiation  of  appropriate  responses  con- 
stitute the  fundamental  work  of  nervous  tissue  (Fig.  30,  A,  D). 
In  some  of  the  lower  Metazoa  the  same  cell  may  do  all  these  tasks. 

84.  Structure. — The  principal   elements   of  nervous  tissue 
are  the  nerve-cells  (ganglion-cells)  and  nerve-fibres.     The  cells, 
which  are  the  centres  of  nervous  activity,  are  usually  large  with 
conspicuous  nuclei.     The  fibres  are,   in  their  essential  parts 
merely  outgrowths  of  the  ganglion-cells.     These  outgrowths  are 
of  two  sorts:  the  dendron,  which  is  a  much,  and  irregularly, 
branched  structure ;  and  the  axon,  or  nervous  fibre  proper.     The 
ganglion  with  its  dendrons  and  axons  make  up  a  neuron.     It  is 
believed  that  the  whole  nervous  system  even  in  the  higher  ani- 
mals is  merely  a  system  of  neurons  in  connection.     The  impulses 
can  go  in  only  one  way :  into  a  cell  by  way  of  dendrons  and  out  of 
it  by  way  of  axons.     From  the  axon  of  one  cell  they  may  pass 
to  the  dendron  of  the  next;  but  not  in  the  opposite  direction. 
Each  cell  may  have  one  or  more  processes  arising  from  it.     These 
fibres  may  pass  just  as  they  arise  from  the  cells,  without  special 
structural  modification,  to  their  connection^.     Such  are  called 
non-medullated  or  gray  fibres.     There  are  usually  however  one  or 
more  protective  sheaths  formed  about  this  essential  axis:  (i) 

Questions  on  Fig.  30. — What  are  the  principal  points  of  contrast  between  the 
plain  and  the  cross-striate  muscular  fibres?  Enumerate  the  principal  regions  of 
the  nerve  cell  figured.  How  does  it  differ  from  a  typical  cell  in  form?  What  are 
the  principal  parts  of  the  nerve-fibre  (D)?  What  are  the  supposed  functions  of 
these  various  portions  ?  Why  is  it  necessary  for  nerve  cells  to  be  in  connection 
with  other  kinds  of  cells?  What  are  the  differences  between  the  contracted  and 
uncontracted  muscle  fibre  (B  and  C)  ?  What  is  meant  by  a  neuron? 


62  ZOOLOGY 

the  medullary  sheath,  consisting  of  a  framework  filled  with  a 
fatty  material,  surrounded  by  (2)  Schwann's  sheath,  a  homoge- 
neous sheath  with  occasional  nuclei  along  its  course  (Fig.  30,  D). 
Fibres  possessing  the  medullary  sheath  are  called  medullated 
or  white  fibres. 

A  nerve  cell  together  with  its  processes  is  called  a  neuron. 
The  whole  nervous  system  may  be  considered  as  made  up  of  such 
units,  which  connect  with  each  other  by  the  delicate  terminal 
branches  of  the  outgrowths.  See  Fig.  36. 

85.  Origin  of  Nervous  Tissue. — Nervous  tissue  always  arises  from  the  ectoderm 
of  the  embryo,  so  far  as  we  know.  In  some  of  the  lower  forms  of  animals,  as  the 
Coelenterata,  the  nervous  cells  may  be  derived  individually  from  the  epithelium. 
In  such  instances  they  have  a  close  connection  with  those  muscle  elements  which 
are  also  of  epithelial  origin  (see  §  82).  In  the  higher  forms  the  origin  of  the  nervous 
matter  from  the  ectoderm  is  somewhat  less  direct  but  essentially  similar.  The 
connection  of  the  nervous  centres  with  the  muscles  and  glands,  etc.,  in  the  higher 
animals  is  a  secondary  condition  and  is  the  result  of  the  growth  of  the  nerve  fibres 
from  the  centres  toward  such  organs.  .  What  directs  their  growth  to  the  right  place 
when  the  fibres  begin  to  grow,  we  do  not  know. 

86.  Summary. 

1.  The  individual  becomes  complex  by  the  increase  of  the 
number  of  cells,  and  by  their  differentiation. 

2.  A  tissue  consists  of  a  group  of  similar  cells  with  their 
products,  which  are  adapted  to  the  performance  of  special  work 
or  function. 

3.  Tissues  differ  morphologically  in  respect  to  the  form,  ar- 
rangement,  and  structure  of  the  cells,   and  in  the  amount, 
arrangement  and  consistency  of  the  intercellular  substance. 

4.  Physiological   differentiation   accompanies   the   morpho- 
logical, the  division  of  labor  becoming  very  complete  in  the 
higher  forms.     The  physiological  value  of  a  tissue  may  depend 
either  upon  the  cells  or  the  intercellular  substance. 

5.  Tissues  may  be  classified  as  follows: 
A.  The  vegetative  or  passive  tissues. 

I.  Epithelial:— 

function: — protection,     absorption,     secretion, 

sensation,  reproduction,  etc. 
kind: — pavement,   columnar,  ciliate,  glandular, 
sensory,  muscular,  reproductive,  etc. 


CELLULAR   DIFFERENTIATION  63 

II.  Supportive  or  connective: — 

function: — binding,  support,  protection, 
character : — abundant  intercellular   substance, 
form: — vesicular,  gelatinous,  fibrous,  cartilagi- 
nous, osseous,  nutritive  (blood  and  lymph) r 
etc. 
B.  The  active  tissues. 

III.  Muscular: — 

function: — irritability,  especially  to  nervous 
stimuli ;  contraction  in  a  definite  direction. 

form: — plain  and  cross-striate  (depending  on 
the  differentiation  of  the  contractile  sub- 
stance.) 

IV.  Nervous: — 

function: — reception       of       general       stimuli, 
transmission     of     impulses,    interpretation, 
and  the  initiation  of  appropriate  responses, 
form: — central    cells    (ganglion)    with    fibrous 

branches  (axon,  dendron). 

6.  The  epithelial  tissues  arise  from  ectoderm,  entoderm  and 
mesoderm;  the  connective  tissue,  from  mesoderm;  the  muscu- 
lar, chiefly  from  mesoderm;  and  the  nervous  tissue,  from 
ectoderm. 

87.  Exercises  for  the  Laboratory  (these  may  be  made  as 
extensive  as  time  and  facilities  will  allow). 

i.  Temporary  demonstrations  of  the  simpler  tissues  should 
be  made  by  the  teacher  or  pupil,  by  teasing  out  with  needles 
small  portions  of  the  appropriate  material  in  a  drop  of  water  on 
the  slide. 

(a)  Blood. — Compare  that  of  earth-worm  or  insect,   frog, 
man.     Place  a  drop  of  fresh  blood  on  the  slide,   and  cover. 
Examine  at  once.     The  teacher  should  have  a  permanent  prepa- 
ration of  the  blood  of  the  frog,   stained  to  show  nucleus  of 
corpuscles. 

(b)  Epithelium. — Mesentery  of  cat;  film  shed  from  skin  of 
frog  kept  a  few  days  in  captivity;  cells  scraped  from  the  eso- 
phagus of  a  recently  killed  frog. 


64  ZOOLOGY 

(c)  Connective    Tissue. — Found    surrounding    muscle,     i.e., 
lean  meat.     Compare  tendon. 

(d)  Muscle. — From  wall  of  stomach,  from  heart,  from  skele- 
tal muscles. 

(e)  Nerve  Fibres. — Small  portion  of  nerve  of  frt>g  or  cat. 

2.  The  teacher  should  secure  permanent  mounts  of  sections 
of  cartilage,  bone,  and  tooth  showing  dentine  and  enamel. 
Properly  stained  preparations  of  glandular  tissue,  of  nerve  cells 
and  their  branches,  and  of  reproductive  epithelium  (see  appen- 
dix), will  greatly  assist  the  pupils  in  securing  an  accurate  idea  of 
these  tissues  and  their  work. 


CHAPTER  VI 

THE   GENERAL  ANIMAL  FUNCTIONS,   AND   THEIR   APPROPRIATE 

ORGANS 

88.  Protoplasmic  Functions. — It  has  already  been  stated 
that  in  protoplasm  reside  the  fundamental  powers  belonging  to 
living  things.     Through  its  agency  all  the  vital  processes  are 
performed.     Through  the  activity  of  its  ferments,  foods  undergo 
changes  that  prepare  them  to  be  used  in  the  manufacture  of 
protoplasm  and  other  complex  cell-substances.     It  alone  has  the 
power  to  assimilate  or  build  up  these  foods  into  the  living  state. 
In  protoplasm  occur  the  oxidation  and  other  chemical  changes 
which  result  in  the  manifestation  of  energy,  as  heat,  motion, 
light,  etc.,  accompanied  by  the  formation  of  waste  products 
which   are   to   be   eliminated.     The   power   of   receiving   and 
responding  to  surrounding  influences,  which  we  have  called  irrita- 
bility and  contractility,  is  likewise  a  power  of  protoplasm.     Out  of 
these  arises  the  possibility  of  organisms  becoming  adapted  to  their 
surroundings.     It  is  not  yet  possible  certainly  to  localize  all  these 
functions  within  the  individual  cell,  although  it  seems  probable 
that  in  some  degree  even  such  protoplasm  as  is  found  in  the 
Amoeba  has  localized  functions.     In  many  single-celled  animals 
there  is  a  considerable  localization. 

89.  Division  of  Labor. — As  the  protoplasmic  units,  i.e.,  the 
cells,  increase  in  number  by  cell  division  and  form  large  masses, 
they  are  no  longer  subjected  to  the  same  influences,  and  are  not 
equally  favorably  situated  for  the  performance  of  all  the  original 
functions.     The  protoplasm  of  all  cells  retains  the  power%  of 
using  food  and  of  building  up  their  own  substance,  but  we  find 
certain  activities  largely  given  over  to  special  groups  of  cells; 
e.g.,  secretion  is  specially  noticeable  in  some,  contractility  in 
some,    and  irritability  in   others.     This   division   of  labor,   is 
accompanied  by  a  corresponding  differentiation  of  structure 
which  constitutes  an  adaptation  to  the  special  work  to  be  done, 

s  65 


66  ZOOLOGY 

and  is  of  great  advantage.     We  have  described  these  structure- 
groups  as  tissues  (see  Chapter  V) . 

90.  Organs. — The  tissues  which  have  been  described  are 
never  independent,  but  are  associated  with  each  other  in  the  per- 
formance of  a  common  function,  to  form  an  organ.     In  each 
organ  there  is  usually  a  principal  tissue  which  determines  its 
function  (as  muscular  tissue  in  muscle,  or  the  glandular  tissue 
in  glands),  and  one  or  more  accessory  tissues  for  support  or  con- 
trol (as  connective  or  nervous  tissue  in  the  organs  mentioned) . 
To  accomplish  some  of  the  activities,  in  the  higher  animals 
especially,  several  organs  of  a  similar  kind  must  work  together. 
These  are  sometimes  spoken  of  collectively  as  systems  of  organs, 
e.g.,  digestive  system,  circulatory  system,  and  the  like. 

91.  Classification  of  the  Systems   of    Organs   and  Func- 
tions.— The  work  that  needs  to  be  done  by  an  organism  may  be 
considered  under  the    following    heads:   (i)    metabolism — in- 
cluding   digestion,    circulation,    assimilation,    respiration,    and 
excretion;  (2)    protection   and   physical   support;  (3)    growth; 
(4)    reproduction;  (5)    movement;  (6)    sensation.     Eight  sys- 
tems of  organs  may  be  distinguished  by  which  this  work  is  done. 
They  are  (i)  the  digestive;  (2)  circulatory;  (3)  respiratory;  (4) 
excretory;  (5)   skeletal  and  integumentary;  (6)   reproductive; 
(7)  muscular,  and  (8)  nervous. 

92.  Metabolism    (Nutrition). — Metabolism    embraces    two 
sets  of  processes,  (i)  constructive  or  anabolic,  known  as  assimi- 
lation, and  (2)  destructive  or  katabolic.     By  constructive  we 
mean  all  the   building-up  processes    in    the    organism   which 
result  in  the  storing  of  food  and  energy,  in  growth,  repair,  and 
reproduction.     We  class  as  destructive  all  those  processes  by 
which  the  complex  cell  substances  are  broken  down  or  rear- 
ranged, and  energy  set  free,  leading  to  change  of  temperature,  to 
nervous  or  muscular  action,  to  secretion  and  excretion.     In  the 
higher  animals  the  nutritive  process  is  a  very  complicated  one 
and  demands  the  cooperation  of  numerous  organs.     It  embraces 
the  ingestion  or  taking  in  of  food;  the  digestion  of  food;  its 
absorption  from  the  digestive  tract  into  the  body  fluids — blood 


THE   GENERAL   ANIMAL  FUNCTIONS  67 

and  lymph;  and  its  transportation  in  these  systems,  which  is 
made  necessary  by  the  fact  that  digestion  is  confined  to  a  special 
region.  It  likewise  includes  the  further  absorption  of  these 
materials  from  the  blood  and  lymph  by  the  cells  for  whose  bene- 
fit all  the  preceding  work  has  been  done;  the  assimilative  process 
within  the  cell  whereby  the  food  material  is  made  into  proto- 
plasm or  other  complex  cell-products;  the  reception  and  trans- 
mission of  oxygen,  by  the  combining  power  of  which  (oxidation; 
see  §26)  these  complex  substances  are  broken  down  into  simpler 
ones — useful,  useless,  or  hurtful  to  the  animal  economy.  Finally, 
the  elimination  of  the  products  of  this  oxidation  or  burning  is  a 
necessary  part  of  the  nutritive  process.  If  the  material  elimi- 
nated from  the  cell  is  of  further  use  the  process  is  known  as 
secretion,  if  not,  excretion.  It  is  undesirable  to  attempt  to  make  a 
sharp  distinction  between  excretion  and  secretion.  Most  so- 
called  secretions  are  really  excretions  from  the  point  of  view  of 
the  protoplasm  which  produces  them. 

93.  The  Digestive  System. — The  simplest  condition  of 
the  digestive  tract  is  found  in  the  gastrula  (archenteron,  Fig. 
13,  4)  or  in  Hydra  (Fig.  81).  Here  there  is  only  one  cavity 
in  the  body  and  the  food  is  taken  up  immediately  by  the  cells 
needing  it.  A  simple  modification  of  this  condition  is  seen 
in  Fig.  31.  A  still  more  complicated  condition  is  shown  in 
Fig.  93.  In  this  form  which  we  may  take  as  the  type,  the 
digestive  tract  is  a  tube,  running  through  the  body,  lined  with 
its  own  epithelium  and  is  separated  from  the  body  wall  by 
the  coelom  or  body  cavity.  The  tube  itself  may  have  any 
degree  of  complexity,  but  consists  essentially  of  (i)  an  an- 
terior portion  (stomodczum)  lined  with  ectoderm,  (2)  a  pos- 
terior portion  (proctod&um)  also  lined  with  ectoderm,  and 
(3)  a  middle  portion  (mesenteron)  lined  with  entoderm.  The 
stomodaeum  or  mouth  region  is  usually  supplied  with  devices 
for  the  capture  and  ingestion  of  the  food.  The  mesenteron 
is  the  true  digestive  region.  It  is  supplied  with  cells  which 
secrete  materials  which  act  upon  foods  in  such  a  way  as  to 
render  them  capable  of  being  absorbed  through  the  entodermic 
cells  into  the  body  cavity,  or  into  that  special  portion  of  it  known 


68  ZOOLOGY 

as  the  circulatory  system.  Pouches  and  outgrowths  from  the 
wall  of  the  mesenteron  are  of  common  occurrence.  These 
serve  to  increase  the  glandular  or  secreting  surface,  the  absorbent 
surface,  and  also  to  retain  the  food  longer  in  contact  therewith 
by  retarding  its  passage  through  the  canal.  The  removal  of 
the  digested  food  from  the  canal  may  be  effected  by  absorption 
or  by  the  active  engulfing  of  food  by  the  entodermal  cells,  much 
as  is  done  by  the  amoeba. 


c  v 


g 

FIG.  31.  Stenostoma  (after  Hertwig).  In  this  Turbellarian  the  digestive  tract  (d.t.)  is  a  blind 
sac.  st.,  boundary  of  stomodseum  and  mesenteron;  c,  cilia;  g,  ganglion  (brain);  g't  ganglion  of  a 
new  individual  which  is  being  formed  by  fission;  o,  mouth;  o',  mouth  of  new  individual  in  process 
of  formation;  w,  excretory  system. 

Questions  on  the  figure. — How  much  of  this  digestive  tract  is  lined  with 
ectoderm?  Which  portion  with  entoderm?  Is  there  a  proctodaeum?  What  are 
the  evidences  that  the  worm  is  in  process  of  division?  Compare  this  digestive 
tract  with  those  in  Figs.  81,  87,  95,  101. 

94.  The  Respiratory  System  and  Function. — In  addition 
to  its  other  food  requirements,  all  protoplasm,  in  proportion 
to  its  activity,  must  have  free  oxygen.  This  is  obtainable  from 
the  air  or  from  the  oxygen  dissolved  in  water.  Oxygen,  being 
a  gas,  must  enter  the  system  in  a  somewhat  different  way  from 
that  by  which  fluids  and  solids  are  ingested.  It  is  best  obtained 
by  absorption  through  moist,  thin-walled  membranes.  Such 
surfaces,  in  connection  with  which  blood  vessels  are  usually 
found,  constitute  the  respiratory  organs.  Any  exposed  sur- 
face meeting  these  requirements  may  serve  as  such.  The 
general  surface  of  all  animals  is  respiratory  in  some  degree. 
In  the  more  complex  animals,  however,  special  additional 


THE  GENERAL  ANIMAL  FUNCTIONS 


69 


organs  must  be  provided.  This  may  be  effected  by  thin  out- 
growths of  the  body  surfaces,  which  are  especially  adapted 
to  water  forms  and  are  called  gills  or  branchics  (Fig.  32);  or  a 
similar  increase  may  be  attained  by  pits  or  ingrowths  of  the 
body  surface,  suited  to  get  oxygen  from  the  air.  Such  are 
called  lungs  or  trachea  (Fig.  33).  Carbon  dioxid,  a  gaseous 
waste  product  resulting  from  the  union  of  oxygen  with  carbon 


FIG.  32. 


FIG.  33. 


b.  c. 


FIG.  32.  Diagram  illustrating  gills  or  branchiae,  b.c.,  cavity  in  which  the  body  fluids  crciu- 
late;  br.,  branchial  filaments  which  are  merely  much  thinned  out-pocketings  of  the  body  wall  (IP); 
ex,  the  external  medium — water — in  which  the  oxygen  is  dissolved. 

Questions  on  the  figure. — What  are  the  essential  features  of  gills  as  suggested 
by  this  figure?  Why  are  they  better  suited  to  water  than  to  air? 

FIG.  33.  Diagram  illustrating  lungs  or  tracheae,  b.c.,  the  cavity  in  which  the  body  fluids 
circulate;  /,  the  walls  of  the  lung,  which  are  much  thinned  in-pocketings  of  the  body  wall  (a>);  ex., 
the  external  medium — usually  the  atmosphere — in  which  the  oxygend  is  foun. 

Questions  on  the  figure. — What  are  the  essential  features  of  lungs  as  suggested 
by  the  figure?  Why  are  such  organs  better  suited  to  aerial  than  to  aquatic  life? 
In  what  respects  are  gills  and  lungs  better  than  the  mere  body- wall  for  the  exchange 
of  the  gases  ? 

which  takes  place  in  the  tissues,  is  ecnomically  eliminated  by 
the  same  organ  which  admits  the  oxygen,  inasmuch  as  the  en- 
trance of  one  gas  is  not  retarded  by  the  outward  passage  of  the 
other.  This  double  process  constitutes  respiration,  although 
the  latter  half  is  also  appropriately  described  as  excretory.  The 
surface  devoted  to  the  exchange  of  the  gases  and  the  special 
devices  necessary  to  renew  the  air  or  water  make  up  the  respira- 
tory system.  The  respiratory  organs  are  frequently  associated 
with  the  anterior  or  posterior  end  of  the  digestive  tract.  As  in 


70  ZOOLOGY 

the  case  of  other  necessary  substances,  the  blood  is  the  vehicle 
by  which  oxygen  is  distributed  from  the  gills  or  lungs  to  the  parts 
of  the  body  needing  it.  The  student  will  realize  that  this  is 
only  the  first  step  in  respiration.  The  real  respiration  takes 
place  in  the  protoplasm  of  the  individual  cells. 

95.  The  Circulatory  System  and  Function. — In  such  con- 
ditions as  are  shown  in  Fig.  81,  there  is  no  circulatory  system. 
The  digested  food  is  merely  distributed  from  cell  to  cell.  In 
animals  in  which  the  digestive  apparatus  is  well  developed, 

FIG.  34- 


FIG.  34.  A  scheme  to  represent  the  circulation  of  the  blood,  in  its  essential  features.  The 
arrows  indicate  the  course  of  the  blood,  a,  arteries;  aur.,  auricle  or  receiving  portion  of  the  heart; 
d,  digestive  tract;  c.  d.,  capillaries  of  the  digestive  tract;  c.r.,  capillaries  of  the  respiratory  organs; 
c.s.,  capillaries  of  the  system;  va.,  valves;  ve,  veins;  vt.t  ventricle. 

Questions  on  the  figure. — What  portions  of  the  apparatus  are  necessary  to 
secure  circulation?  Which  secure  the  real  objects  for  which  the  circulation 
exists?  Why  are  valves  essential ?  What  common  work  occurs  in  the  three  classes 
of  capillaries  figured  above?  What  special  type  of  work  is  characteristic  of  each  of 
the  three? 

some  device  becomes  necessary  for  the  distribution  of  the  food. 
The  body  cavity  with  its  contained  fluids  may  do  this  work  as 
in  Fig.  31.  Usually,  however,  when  the  mesodermal  layers  be- 
come well  developed,  there  arises  in  connection  with  it  a  series 
of  branching  tubes  containing  special  fluids,  blood  or  lymph. 
These  tubes  by  their  ramifications  connect  the  digestive  sur- 
faces with  the  various  parts  of  the  body.  Some  branches  like- 
wise extend  to  those  special  surfaces  where  the  oxygen  of  the 
external  medium  may  be  had.  Naturally  then  the  complexity 


THE   GENERAL  ANIMAL  FUNCTIONS  71 

and  the  special  structure  of  the  circulatory  system  depend 
largely  upon  the  position  and  degree  of  development  of  the 
digestive  and  respiratory  organs.  In  order  to  secure  the  neces- 
sary motion  of  the  fluids  contained  in  the  tubes,  the  walls  of 
the  latter  are  supplied  with  muscular  fibres,  and  contract  more 
or  less  rhythmically.  If  the  motion  is  to  have  a  definitely  con- 
tinuous direction,  as  is  ordinarily  the  case,  valves  are  usually 
so  placed  that  motion  in  the  opposite  direction  will  be  im- 
possible. The  (one  or  more)  contractile  regions  are  called 
hearts;  vessels  conducting  blood  from  the  heart  are  arteries, 
those  carrying  blood  toward  the  heart,  veins.  In  the  region 
where  the  vessels  are  smallest  and  have  very  thin  walls,  the 
exchanges  between  the  blood  and  the  other  tissues  occur.  This 
is  the  region  of  capillaries.  The  blood  system  has  capillaries 
in  the  walls  of  the  digestive  tract,  in  the  respiratory  organs, 
in  the  kidneys,  in  the  liver,  and  in  and  about  all  the  tissues  re- 
ceiving a  direct  blood  supply.  The  capillary  region  is  that  for 
which  the  rest  exists;  it  is  the  physiologically  important  part 
of  the  system.  Fig.  34  illustrates  the  arrangement  of  parts 
found  in  a  common  type  of  circulatory  apparatus. 

96.  Demonstration. — Circulation  of  blood  in  tail  of  tadpole;  in  the  web  of  the 
foot  of  a  frog;  or  in  the  fin  of  small  fish.  Distinguish  veins  and  arteries.  Notice 
behavior  of  corpuscles  in  passing  through  small  capillaries.  Compare  rate  of  flow 
in  vessels  of  different  size. 

97.  The  Excretory  System  and  Function. — Beside  the  car- 
bon dioxid  eliminated  from  the  blood  in  the  lungs  or  gills,  other 
waste  products  of  oxidation  are  to  be  removed  from  the  tissues 
where  they  are  produced.  Important  among  these  are  the  nitrog- 
enous wastes,  urea  and  uric  acid.  In  organisms  in  which 
there  is  no  regular  blood  system,  these  waste  products  may  be 
carried  directly  from  the  tissues  to  the  surface  by  a  system  of 
tubes  beginning  as  capillaries.  In  the  majority  of  animals  the 
canals  (nephridia)  pass  from  the  body  cavity  to  the  exterior. 
These  are  seen  in  a  simple  condition  (Fig.  3  5)  in  the  segmented 
worms.  For  a  more  unified  condition  see  unsegmented  worms 
(Figs.  88,  89).  The  kidneys  of  higher  forms  are  considered 
to  be  derived  from  these.  In  the  higher  animals  the  kidneys 


72  ZOOLOGY 

have  a  special  blood  supply  and  the  waste  products  are  extracted 
from  the  blood  while  it  is  in  the  kidney. 

98.  The  Skeletal  System  and  its  Functions. — The  cells  of 
the  body  frequently  excrete  from  themselves  materials  which, 
while  no  longer  of  use  to  the  protoplasm,  are  not  entirely  re- 
moved from  the  body  by  the  blood  and  serve  important  passive 
functions.  These  excretions  or  secretions  may  surround  and 
protect  and  give  rigidity  to  the  cell  itself  (e.g.,  cell- walls; 
shells  in  the  single-celled  animals),  may  bind  the  softer  cells  to- 

FIG.  35- 


PIG.  35.  Diagram  of  a  nephridium  (simple  kidney  tubule)  of  a  Segmented  Worm.  b.,  b'., 
blood  vessels;  c,  ccelom;  d,  duct  of  the  nephridium;  e,  external  opening;  cf,  ciliated  funnel  opening 
into  ccelom;  gl.,  glandular  or  secreting  portion;  s,  septum;  W,  body  wall  composed  of  longitudinal 
muscle  fibres,  circular  fibres,  and  epithelial  layer;  w,  wall  of  gut. 

Question  on  the  figure. — Judging  from  its  relation  to  the  coelom,  to  the  blood 
vessels,  and  to  the  outside  world  what  would  seem  a  reasonable  function  for  the 
nephridium  ? 

gether  into  a  resistant  tissue  (intercellular  substance  in  bone,  etc.) , 
or  may  be  secreted  at  the  surface  of  the  organism  as  a  whole 
(cuticula  in  insects  and  shell  in  mollusks).  The  hard  parts 
serve  primarily  for  the  support  and  protection  of  the  softer 
tissues.  Incidentally  they  come  to  serve  a  very  important  use 
as  points  of  attachment  for  muscles.  The  skeleton  may  be 
external  (the  integumentary  skeleton,  or  eocoskeleton)  as  in 
crayfish,  or  internal,  as  the  endo skeleton  of  vertebrates.  In 
many  instances  both  kinds  of  skeletal  structures  may  occur 
simultaneously,  yet  it  is  usually  true  that  if  the  exoskeleton 
is  well  developed  the  endoskeleton  will  be  poorly  represented. 
Each  has  important  advantages  and  limitations.  To  allow 


THE    GENERAL   ANIMAL   FUNCTIONS  73 

motion  as  the  result  of  muscular  action  the  skeleton,  if  rigid, 
evidently  must  be  in  segments  and  jointed.  The  articulation,  or 
joint,  is  as  important  for  motion  as  the  harder  parts.  Clearly 
the  muscles,  to  do  any  work  on  the  skeleton,  must  attach  on 
opposite  sides  of  the  joint. 

99.  Growth. — There  are  no  special  organs  of  growth,  yet 
growth  is  one  of  the  most  immediate  and  important  manifesta- 
tions of  the  nutritive  process.     Growth  is  to  be  defined  as  in- 
crease in  volume  or  mass  and  may  result  from  either  or  all  of 
three  processes:  viz.  (i)  absorption  of  water,  (2)  formation  of 
protoplasm  and  the  multiplication  of  cells,  and  (3)  formation  of 
non-protoplasmic  cell-products,  either  within  or  among  the  cells. 

The  rate  and  character  of  growth  are  modified  by  such  ex- 
ternal conditions  as  temperature,  light,  quantity  and  quality 
of  the  food  supply,  etc.  Growth  does  not  continue  indefinitely. 
Its  continuance  is  determined  by  the  relation  of  the  anabolic 
to  the  katabolic  processes  in  the  body.  The  time  conies  in  the 
life  of  every  complex  organism  where  the  income  no  longer 
equals  the  outgo,  and  growth  must  cease.  Later  still  the  wear 
is  not  made  good  by  the  income,  and  death  results.  Just  what 
causes  organisms  to  cease  growing  we  do  not  know. 

100.  Reproduction   and   the   Reproductive    Organs. — Since 
individual  organisms  are  limited  both  with  regard  to  growth 
and  length  of  life,  it  is  apparent  that  a  given  class  of  forms 
cannot  continue,  unless  some  method  of  originating  new  in- 
dividuals be  found.     This  production  of  new  individuals  by 
the  instrumentality  of  the  old  is  reproduction.     In  many  of 
the  lower  animals  this  is  merely  a  growth  process, — "growth  be- 
yond the  limits  of  the  individual."     In  the  single-celled  ani- 
mals reproduction  means  the  formation  of  the  protoplasm  into 
two  or  more  masses,  by  dividing  into  two  equal  parts  (divi- 
sion), by  breaking  into  a  large  number  of  sub-equal  portions 
(fragmentation),  or  by  budding  (Chapter  III,  §40).     In  bud- 
ding there  is  the  formation  of  a  local  outgrowth  which  ulti- 
mately attains  the  size  and  character  of  the  parent.     In  division 
the  resulting  individuals  cannot  be  distinguished  as  parent  and 
offspring.     Such  reproduction,   involving  only  one  parent,   is 


74  ZOOLOGY 

asexual.  It  usually  occurs  when  the  adult  size  of  the  animal 
is  attained.  It  is  not  confined  to  the  Protozoa  or  single-celled 
animals,  but  may  occur  in  several  Invertebrate  groups  in  which 
(Hydra,  Fig.  81)  there  is  not  a  high  degree  of  specialization. 
The  budded  individual  or  offspring  may  in  such  cases  consist 
of  one  cell  or  of  many.  In  addition  to  the  internal  stimulus  af- 
forded by  the  attainment  of  normal  size,  external  conditions 
such  as  diminished  food  supply,  temperature  changes,  etc.,  in- 
fluence the  process  of  non-sexual  reproduction. 

101.  Sexual  Reproduction. — It  seems  for  some  reason  that 
even  in  the  one-celled  animals  the  method  of  reproduction  by 
division  cannot  be  continued  indefinitely  without  some  ill  effects 
to  the  organism.  In  many  Protozoa  there  is  at  certain  times  a 
union  of  two  individuals,  either  temporarily  or  permanently, 
accompanied  by  exchange  of  nuclear  material  or  by  a  fusion  of  the 
whole  protoplasm.  After  a  period  of  rest  and  the  coming  of  fa- 
vorable conditions  of  life,  division  begins  again  with  renewed 
activity.  Something  similar  is  seen  in  the  more  complex  animals 
—the  Metazoa.  After  a  period  of  cell  divisions,  by  which  the 
individual  body  is  built  up,  the  majority  of  cells,  as  muscle  or 
nerve  cells,  appear  to  lose  their  power  of  dividing,  and  even  the 
less  differentiated  cells  which  we  have  described  as  the  ova  and 
sperm,  which  arise  from  the  primordial  germ  cells  (§  49),  are  in- 
capable of  continuing  the  division  necessary  to  produce  a  new 
individual  until  they  have  been  stimulated  by  union  with  each 
other  (or  by  some  artificial  means).  Such  unions  of  cells,  to 
form  by  later  divisions  a  new  individual,  are  called  conjugation 
or  fertilization,  and  the  new  individual  which  results  is  said  to 
arise  by  sexual  reproduction.  The  uniting  cells  may  be  similar 
(as  in  Pandorina),  in  which  case  the  union  is  isogamous.  More 
usually  the  cells  are  different  and  the  union  is  heterogamous. 
In  the  latter  case  the  cells  are  called  ovum  and  sperm  (Chapter 
IV)  and  are  usually  formed  in  different  individuals,  though 
very  often  the  same  individual  may  give  rise  to  both  classes  of 
cells  (hermaphroditism)  from  different  regions  of  the  germinative 
epithelium,  or  in  the  same  organ  at  different  times.  The 
special  organs  in  which  the  ova  are  produced  are  called  ovaries. 


THE  GENERAL  ANIMAL  FUNCTIONS  75 

The  sperm  cells  are  formed  in  testes.  The  individuals  (that 
is,  the  male  and  female)  producing  the  different  classes  of  cells 
are  often  very  different  in  other  respects  also.  This  is  known 
as  sexual  dimorphism  (Chapter  VII,  §149). 

1 02.  Practical  Exercises. — Compare  the  males  and  females  of  the  various  animal 
types  with  which  you  are  familiar.  In  what  groups  of  animals  does  non-sexual 
reproduction  occur?  Give  the  gist  of  Geddes  and  Thompson's  theory  as  to  the 
origin  of  sexuality.  What  is  the  view  based  on  the  discoveries  of  Mendel?  Com- 
pare any  other  theories  available  to  you.  What  are  the  conceivable  advantages 
and  disadvantages  of  the  asexual  method?  of  the  sexual?  of  hermaphroditism ?  of 
sexual  dimorphism? 

103.  Movement  and  the  Muscular  System. — The  desira- 
bility of  motion  in  animals  arises  from  the  necessity  of  seek- 
ing food  and  of  escaping  unfavorable  influences.  These  con- 
ditions constitute  the  most  imperious  stimuli  to  which  the  or- 
ganism is  subject.  We  have  already  seen  (§§20,  24)  that  the 
fundamental  irritability  and  contractility  of  protoplasm  make 
this  response  possible  in  the  simplest  conditions.  In  somewhat 
higher  forms,  specially  developed  protoplasmic  fibrils  appear, 
such  as  cilia,  or  the  fibrils  in  the  -stalk  of  Vorticella,  in  which 
the  power  of  contracting  is  strikingly  manifest  (see  Figs.  68 
and  70).  While  this  is  found  in  Protozoa,  it  is  much  more 
clearly  shown  in  the  muscular  tissue  (Fig.  30)  of  still  higher 
animals.  Locomotion  varies  in  efficiency  in  different  animals 
not  merely  on  account  of  varying  muscular  structure  but  in 
accordance  with  the  arrangement  of  the  hard  parts  to  which 
the  muscle  fibres  are  attached,  and  the  nature  of  the  medium 
which  must  be  penetrated.  Many  aquatic  forms,  though  free- 
swimming  in  their  early  stages,  may  become  attached  and  give 
up  the  power  of  locomotion  in  the  adult  condition.  Such 
attached  or  poorly  moving  forms  ordinarily  secrete  an  external 
shell  or  covering  into  which  they  can  withdraw  for  protection 
(e.g.,  barnacles,  many  polyps).  They  must  depend  upon  cur- 
rents in  the  water  to  supply  them  with  food .  They  are  frequently 
able  to  produce  the  currents  by  the  motion  of  parts  of  the  body. 
The  majority  of  active  movers  have  hard  parts  which  serve 
as  levers  to  which  the  muscles  are  attached.  The  parts  of  the 
skeleton,  which  may  be  either  external  to  the  muscles  or  sur- 
rounded by  them,  articulate  with  one  another  by  a  hinge  or 


76  ZOOLOGY 

movable  joint,  as  illustrated  by  vertebrates  or  insects.  In 
some  forms  without  a  conspicuous  skeleton,  as  the  earthworm, 
there  is  a  der mo-muscular  wall  surrounding  a  fluid-filled  cavity. 
Locomotion  is  effected  in  these  by  the  alternate  use  of  the 
longitudinal  and  circular  fibres,  changing  the  relative  position 
of  the  parts  of  the  body.  The  special  appendages,  particularly 
the  paired  appendages,  are  important  motor  organs  in  nearly  all 
actively  moving  animals. 

104.  Sensation  and  Sensory  Structures. — In  a  simple  bit 
of  protoplasm  it  is  manifest  that  the  differences  between  the 
living  matter  and  the  outside  world  are  greater  than  the  struc- 
tural differences  between  the  parts  of  the  protoplasm  itself. 
Thus  we  would  expect  the  stimuli  arising  from  the  action  of 
environment  upon  the  living  material  to  be  among  the  most 
important  experienced  by  the  organism,  and  that  the  superficial 
protoplasm  by  virtue  of  its  irritability  (see  also  §20)  would  most 
promptly  feel  and  respond  to  such  stimuli.     The  changes  thus 
instituted  will  be  felt  sooner  or  later  to  the  remotest  parts  of  the 
cell  mass.     This  transfer  of  the  effects  of  a  stimulus  through  a 
longer  or  shorter  distance  introduces  us  to  a  second  nervous 
function, — internal  irritability  of  protoplasm  or  conductivity. 

105.  As  an  organism  increases  in  the  number  and  variety 
of  its  cells,  the  specialized  structures  need  to  be  more  com- 
pletely bound  to  one  another.     It  becomes  necessary  not  only 
that  they  receive  impulses  from  such  parts  as  are  favorably 
situated  for  the  reception  of  stimuli,  but  that  a  degree  of  co- 
ordination of  the  interrelated  parts  be  secured,  in  order  that 
just  such  response  shall  be  made  as  will  best  meet  the  needs 
of  the  organism.     This  power  of  coordinated  response  to  ex- 
ternal stimuli  makes  it  possible  for  an  organism  to  become  suited 
to  its  environment. 

1 06.  In  the  higher  forms,   the  work  above  described  de- 
mands five  classes  of  structures  (see  Fig.  36) : — (i)  end  or  sense 
organs,  which  are  specially  sensitive  to  stimuli  of  different  orders, 
as  mechanical  (touch),  chemical,  ethereal  (light),  etc.;  (2)  con- 
ductive tracts  (afferent  nerves'),  which  connect  (i)  with  (3)  cen- 


THE   GENERAL   ANIMAL   FUNCTIONS 


77 


tral  nervous  structures  (ganglion-cells)  where  the  impulse  is  re- 
ceived and  suitable  responsive  impulses  are  originated;  (4)  con- 
ductive tracts  (efferent  nerves),  which  make  the  work  of  the 
central  organs  of  value  by  carrying  an  impulse  which  produces 
corresponding  activities  in  (5),  some  form  of  actively  related 
cells, — muscular,  glandular,  or  nervous.  It  is  readily  apparent 
how  increase  of  volume  and  differentiation  of  the  other  parts  will 
make  necessary  a  more  complicated  nervous  system.  The  spe- 
cial arrangement  of  these  parts  of  a  complete  system  differs  very 

FIG.  36. 


-y. 


PIG.  36.  Scheme  showing  the  essential  relations  of  the  parts  of  a  nervous  system:  i,  the  sensory, 
end  organ  (epithelial);  2,  afferent  nerve  tract;  3,  central  nervous  cells  (ganglia);  4,  efferent  nerves 
leading  to  5,  muscle,  gland,  etc.  g,  ganglion  cells;  gl.,  gland;  m,  muscle  fibre;  «./.,  nerve  fibre;  s.e.. 
sensory  epithelium. 

Questions  on  the  figure. — What  seems  to  be  the  function  of  the  various  parts 
or  elements  in  this  scheme?  Your  reasons  for  your  view? 

much  in  various  animal  groups,  yet  it  may  be  said  that  there  is 
a  progressive  accumulation  of  the  central  nervous  matter  at  the 
anterior  end  of  the  body  as  we  ascend  the  scale  of  animal  life. 
When  this  concentration  is  well  advanced  the  mass  of  nervous 
matter  is  called  the  central  nervous  system  which  always  includes 
the  brain.  The  nerves  passing  to  and  from  the  central  part  and 
their  endings,  taken  collectively,  are  described  as  the  peripheral 
nervous  system. 

107.  Arrangement  of  the  Central  Nervous  System. — The 

ganglion  cells  composing  the  nervous  system  may  be  so  scat- 


ZOOLOGY 


FIG.  37. 


FIG.  37.     Diagram  shoving  arrangement  of  the  nervous  matter  in  Starfish,     c,  ganglionated  ring 

about  the  mouth;  o,  mouth;  r.n.,  radial  nerve  in  each  arm. 

FlG.  38.     The  nervous  system  of  the  Clam, — from  the  dorsal  aspect,     c,  anterior;  o,  mouth;   e.g., 
cerebral  ganglia  (brain);  p.g.,  pedal  ganglia;  v.g.,  visceral  ganglia. 

Questions  on  Fig.  37. — Describe  in  your  own  terms  the  way  in  which  the 
principal  nerve  elements  are  arranged  in  the  starfish?  Compare  it  with  those 
which  follow.  In  what  respects  similar?  In  what  unlike  them? 


FIG.  39. 


FIG.  40. 
CL 


FIG.  39.  Arrangement  of  the  nervous  material  in  the  anterior  end  of  an  Oligochete  Worm, 
seen  in  profile.  That  part  of  the  body  wall  nearest  the  observer  is  supposed  to  be  removed,  a, 
anterior;  b.w.,  body  wall;  g,  dorsal  ganglia  (brain);  g',  ventral  chain  of  ganglia;  n,  nerve  ring  around 
the  pharynx;  o,  mouth;  p,  pharynx. 

FIG.  40.     The  central  nervous  system  in  a  Leech.     Lettering  as  in  Fig.  39. 

Questions  on  figures  38,  39,  and  40. — How  is  the  nervous  matter  related  to  the 
digestive  tract  and  to  the  animal  as  a  whole  in  all  of  these  figures  ?  Compare  with 
figures  in  other  texts.  Is  there  any  apparent  correlation  between  the  form, 
symmetry  or  segmentation  of  the  animal  and  the  arrangement  of  the  nervous 
material?  Can  you  state  your  conclusion  as  a  law? 


THE    GENERAL   ANIMAL   FUNCTIONS  79 

tered  through  the  superficial  layers  as  scarcely  to  deserve  the 
name  ''central"  (Hydra).  The  nerve  cells  may  be  arranged 
in  a  ring  about  the  mouth,  or  gullet,  with  or  without  addi- 
tional bands  of  nervous  tissue,  containing  cells,  passing  radially 
from  it  (as  in  echinoderms  and  some  ccelenterates ;  see  Fig. 
37).  In  the  higher  invertebrates  this  process  of  concentration 
continues  and  the  ganglionic  cells  are  collected  into  two  or  more 
ganglia  connected  by  nerve  fibres  (commissures).  Usually 
a  pair  of  ganglia  occurs  in  the  region  of  the  mouth,  and  dorsal 
to  it  (e.g.,  clam,  Fig.  38).  In  segmented  forms,  as  the  earth- 
worm and  crayfish,  there  is  also  a  series  of  ganglia  connected 
by  fibres,  ventral  to  the  digestive  tract.  This  chain  is  in  turn 
connected  with  the  dorsal  ganglia  by  a  loop  of  fibres  passing 
round  the  esophagus  (Figs.  3  9  and  40) .  In  vertebrates  the  cen- 
tral nervous  system  consists  primarily  of  a  tube  with  nervous 
walls — the  spinal  cord — which  may  be  specially  enlarged  and 
thickened  at  the  anterior  end  to  produce  the  brain  (Fig.  172). 
From  the  various  parts  of  this  cord  the  nerves  take  their  origin, 
and  run  to  all  parts  of  the  body. 

1 08.  The   Peripheral   Nervous    System:  Sense    Organs.— 

We  know  by  experimentation  that  in  the  lowest  animals  even, 
or  for  that  matter,  in  protoplasm,  certain  external  conditions 
produce  definite  responses  or  changes.  We  also  know  that 
these  external  happenings  and  their  responses,  in  our  own 
case,  are  accompanied  by  certain  sensations,  as  touch  or  taste. 
By  inference,  both  from  the  nature  of  the  response  and  from 
the  structure  of  organs,  we  reach  the  conclusion  that  the  lower 
vertebrates  and  higher  invertebrates  experience  sensations  in 
some  degree  similar  to  our  own.  The  classes  of  possible  stimuli 
have  already  been  mentioned  (§21).  Those  producing  in  us 
definite  sensations  are:  simple  contact  stimuli,  producing  the 
sensation  of  touch  and  pressure;  vibratory  contacts,  giving  rise 
to  hearing  and  temperature  sensations;  gravity,  giving  us  our 
sense  of  position  in  respect  to  the  pull  of  gravity;  chemical  ac- 
tions, making  possible  sensations  of  taste  and  smell;  ethereal 
vibrations,  producing  the  sensation  of  light.  In  the  lowest 
forms  of  animals  there  are  no  specialized  organs  for  the  reception 


80  ZOOLOGY 

of  particular  stimuli,  and  in  such  cases  it  is  reasonable  to  infer  that 
the  distinctness  of  the  sensation  cannot  be  very  great.  In 
almost  all  animals,  however,  certain  areas  are  specially  suited 
to  be  stimulated  by  special  stimuli. 

109.  Touch. — There  are  two  principal  ways  by  which  contact  stimuli  are 
received  among  animals.  Fibres  of  the  central  nervous  system  may  pass  to  the 
skin  and  end  among  its  outer  layers  as  free  nerve  terminations,  or  these  fibres  may 
become  intimately  united  with  one  or  more  of  the  cells  of  the  epithelium.  The 
most  common  of  the  tactile  organs  in  vertebrates  are  of  the  first  class.  Where  the 
stimulus  reaches  the  nerve  through  a  nervous  epithelium,  the  epithelial  cells  often 
have  special  developments  such  as  hairs,  bristles,  and  the  like,  whereby  the 
possibility  of  contact  with  external  objects  is  increased  (Fig.  19,  A,  s).  The 
appreciation  of  changes  in  temperature  is  also  associated  with  the  general  skin 
surface. 

no.  Chemical  Sense  (including  taste  and  smell). — It  is  impossible  for  us  to 
distinguish  between  taste  and  smell  in  the  lower  animals.  Indeed  it  is  with 
difficulty,  in  some  instances,  that  we  separate  the  sensations  obtained  from  the  two 
sources  even  in  our  own  case.  Almost  all  animals  seem  to  have  some  power  of 

FIG.  41. 


s.c. 


FIG.  41.     Statocyst  in  a  Mollusk.     n,  nerve;  o,  statolith;  s.c.,  sensory  cells  in,  wall  of  statocyst. 

(After  Claus.) 

Questions  on  the  figure. — What  immediately  stimulates  the  sensory  epithelium 
in  this  case?  What  kinds  of  general  agencies  might  be  supposed  to  produce  the 
necessary  motion  for  this  purpose?  What  is  the  present  view  of  the  function  of 
statocysts? 

appreciating  the  chemical  condition  of  the  medium  in  which  they  live.  In  aquatic 
animals  the  chemical  sense  organs  may  be  distributed  over  the  surface  of  the 
body.  In  the  higher  animals  they  collect  more  and  more  at  the  anterior  or  mouth- 
end  of  the  animal,  with  manifest  advantage  to  the  animal.  In  the  higher  land 
forms,  especially  the  vertebrates,  the  organs  of  the  chemical  sense  come  to  lie  in  or 
about  the  mouth  and  nose, — the  beginnings  of  the  digestive  and  respiratory  tracts 
respectively.  These  senses  are  specially  related  to  the  testing  of  food  and  the 
medium  in  which  the  animal  lives.  For  this  purpose  their  position  in  the  mouth 


THE    GENERAL   ANIMAL   FUNCTIONS 


8l 


and  nose  is  especially  favorable.     The  senses  thus  far  enumerated  seem  among 
the  earliest  developed  in  the  animal  kingdom. 

in.  Hearing  and  Equilibrium  Sense. — It  is  by  no  means  certain  that  the  lowei 
animals  possess  the  ability  to  appreciate  those  vibrations  in  matter  (air,  water, 
etc.),  which  arouse  in  us  the  sensation  of  sound.  There  are  in  several  groups  of 
such  animals  organs,  the  structure  of  which  would  suggest  that  they  might  receive 
vibrations  of  the  medium  in  which  they  live.  In  their  simplest  condition  they 
consist  of  a  sac  (otocyst  or  statocyst)  derived  from  the  ectoderm  and  lined  by  an 
epithelium  containing  sensory  cells.  From  these  cells  sensory  hairs  extend  into 

FIG.  42. 


FIG.  42.      Antenna  of  Male  Mosquito  (Culex  pipiens).     By  J.  W.  Folsom. 

Questions  on  the  figure. — Compare  with  the  antennae  of  a  female  (see  Fig.  65). 
What  are  the  differences  between  the  head  of  the  male  and  female  mosquitoes? 
What  is  believed  to  be  the  function  of  these  plumose  antennas?  What  are  the 
evidences  for  this  view? 

the  cavity  (Fig.  41).  The  cavity  contains  a  fluid  which  may  support  one  or  more 
solid  particles  (statoliths} .  With  the  vibration  of  the  medium  the  whole  would  be  put 
into  vibration,  but  the  inertia  of  the  contained  fluid  and  statoliths  would  cause  the 
latter  to  strike  against  the  hairs  and  thus  serve  as  stimuli  to  the  sensory  cells. 
Late  researches  tend  to  prove  that  these  structures  are  organs  enabling  the  organism 
to  appreciate  the  pull  of  gravity  and  movements  in  the  water  rather  than  to  hear. 
In  higher  forms  the  ear  becomes  immensely  more  complex,  but  the  general  condi- 
tions both  of  origin  and  structure  appear  to  be  much  as  described  for  the  statocysts. 
That  is  to  say,  the  final  sensory  cells  are  ectodermal  in  origin,  but  now  line  a  sac 
deeply  imbedded  in  the  tissues  of  the  skull.  In  some  of  the  lower  animals,  as  insects, 
6 


82 


ZOOLOGY 


there  are  also  external  vibratile  hairs  and  vibrating  membranes  which  are  auditory 
(Fig.  42). 

112.  Sight. — There  are  three  distinct  facts  to  be  noted  with  respect  to  visual 
sensation  in  the  higher  forms  of  animals:  the  perception  of  light,  the  perception  of 
color  (i.  e.,  light  of  different  wave-frequency)  and  the  formation  of  images  of  ex- 
ternal objects.  It  has  already  been  seen  (§21)  that  protoplasm  is  sensitive  and 
responsive  to  light  without  any  special  organs.  The  simplest  visual  organs  found 
in  multicellular  animals  consist  merely  of  epithelial  cells  containing  pigment  in 


FIG.  43.  Diagrams  showing  some  of  the  stages  in  the  increasing  complexity  of  the  simple  eye 
in  Invertebrates.  A,  simple  pigment  spot  in  epithelium  having  nerve-endings  associated  with 
pigment  cells  (as  in  some  medusae);  B,  pigment  cells  in  a  pit-like  depression  (as  in  Patella};  C,  with 
pin-hole  opening  and  vitreous  humor  in  cavity  (as  in  Trochus);  D,  completely  closed  pit,  with  lens 
and  cornea  (as  in  Triton  and  many  other  Mollusks);  E,  pigment  area  elevated  instead  of  depressed, 
lens  of  thickened  cuticula  (as  in  the  Medusa,  Lizzta);  F,  retinal  cells  more  highly  magnified,  ep., 
epidermis;/,  nerve  fibre;  /,  lens;  op,  optic  nerve;  p,  pigment  cells;  r,  retina;  v.h.,  vitreous  humor. 

Questions  on  the  figures. — What  changes  take  place  in  the  sensory  epithelium 
in  this  series  of  figures?  What  is  gained  by  such  a  depression  as  occurs  in  B? 
What  purpose  is  served  by  the  pinhole  and  the  vitreous  humor  of  C?  Describe 
the  change  from  C  to  D.  What  is  gained?  What  may  be  the  function  of  the 
pigment?  Compare  texts.  In  what  respects  does  E  differ  from  the  other  types? 
What  two  types  of  cells  are  figured  as  belonging  to  each  retina?  What  constitutes 
the  retina? 

which  changes  are  wrought  apparently  by  the  action  of  light  (Fig.  43,  a).  These 
changes  affect  the  nerve  fibres  associated  with  the  pigment  cells  and  thus  the 
central  nervous  organ.  Such  eyes  are  capable  only  of  giving  knowledge  of  the 
intensity  or,  if  properly  constructed,  of  intensity  and  direction  of  the  light  and  do 
not  form  an  image  of  external  objects.  There  are  several  types  of  image-forming 
eyes  in  the  animal  kingdom.  The  most  familiar  of  these  is  the  "camera  eye"  of 
vertebrates,  so  called  because  it  illustrates  the  principles  made  use  of  in  the 


THE    GENERAL   ANIMAL   FUNCTIONS 


construction  of  the  photographic  camera.  In  this  there  is  a  lens  or  body  which 
refracts  the  rays  of  light  in  such  a  way  that  all  the  rays  passing  from  a  point  of  the 
object  are  brought  to  a  focus  at  a  point  on  the  retina.  Another  type  of  image- 
forming  eye  is  the  compound  eye  of  insects  and  Crustacea  (Fig.  44).  These  are 
made  up  of  a  large  number  of  eye  elements — each  structurally  complete  in  itself — 
whose  separately  formed  images  must  nevertheless  be  joined  in  order  to  form  a 
picture. 


FIG.  44. 


— c 


•—£/,. 


JL n 


PIG.  44.  Diagram  illustrating  the  compound  eye  of  arthropods.  A,  the  whole  eye  shown  in 
section;  B,  one  of  the  eye-elements  (ommatidium)  more  highly  magnified,  c,  cuticular  facets;  ep, 
epidermis;  /,  group  of  cells  forming  lens-like  body;  n,  optic  nerve  fibres;  o,  optic  ganglia;  p,  pigment 
cells. 

Questions  on  the  figures. — In  what  way  is  the  independence  of  each  ommatidium 
secured?  In  other  words  what  is  to  prevent  the  light  which  comes  in  obliquely 
from  passing  from  one  ommatidium  to  another?  In  what  conceivable  way  is  a 
general  image  obtained  from  these  various  partial  views?  What  groups  of  animals 
possess  eyes  of  this  sort?  Compare  the  diagram  B  with  the  figure  of  the  complete 
ommatidium  of  the  lobster  (Fig.  127). 

The  degree  to  which  the  color-sense  is  developed  among  lower  animals  is  very 
uncertain.  The  simplest  animals  may  respond  differently  to  light  of  different 
colors,  but  this  is  a  very  different  thing  from  saying  that  they  possess  the  color- 
sense  in  the  meaning  that  we  give  to  the  word. 

To  summarize, — the  essential  part  of  the  eye  is  the  sensitive  layer  known  as  the 
retina.  The  other  parts  of  the  complex  eye-structure  serve  the  purposes  of  shutting 
out  the  light  except  from  certain  directions;  of  focusing  the  light  admitted  in  such  a 
way  as  to  increase  its  intensity  and  form  an  image  on  the  retina;  of  adjusting  this 
apparatus  to  objects  at  different  distances;  of  nourishing  and  supporting  the  more 
important  portions  of  the  apparatus;  and  of  moving  the  eye  so  as  to  take  into  view 
different  portions  of  the  surroundings.  Some  of  the  various  grades  of  complexity 
of  eye-structure  in  the  invertebrate  series  beginning  with  a  pigment  spot  and 
ending  with  a  complete  lens-eye,  are  shown  in  Fig.  43. 


84  ZOOLOGY 

113.  Analogy  and  Homology. — In  comparing  various  ani- 
mals we  find  that  they  may  do  the  same  work  with  organs  that 
arise  in  very  different  ways,  which,  however,  because  they  are 
adapted  to  perform  similar  tasks,  look  somewhat  alike.  Such 
structures  are  said  to  be  analogous  (as  the  wing  of  a  bird  and  the 
wing  of  a  butterfly).  In  other  cases  organs  that  originate  in 
the  same  way  may  have  been  used  so  differently  as  to  have  a 
very  different  appearance,  as  the  various  "legs"  of  the  crayfish, 
or  the  wing  of  a  bird  and  the  arm  of  man.  These,  notwithstand- 
ing their  superficial  differences,  are  said  to  be  homologous  because 
of  the  fundamental  equivalency  of  structure. 

FIG.  45- 


FIG.  45.  Diagrams  illustrating  two  stages  in  the  development  of  the  vertebrate  eye.  A, 
showing  the  relation  of  the  ectoderm,  the  brain  vesicle,  and  the  optic  vesicle.  The  right  side  of  the 
figure  shows  a  later  stage  than  the  left.  B,  later  stage,  showing  the  lens,  eye-ball  and  retina  in 
position,  b.v.,  brain  vesicle  formed  by  the  invagination  of  the  ectoderm  (ecf.);  I,  lens;  mes.,  meso- 
dermal  tissue;  o.n.,  optic  nerve;  o.s.,  optic  stalk;  o.v.,  optic  vesicle,  a  portion  of  the  brain  vesicle; 
r,  retinal  layer;  v.h.,  interior  of  eye-ball  which  comes  to  contain  the  vitreous  humor. 

Questions  on  the  figures. — Which  portions  of  the  eye  are  derived  directly  from 
the  ectoderm?  Which  indirectly,  i.e.,  from  the  brain?  Which  portions  seem  of 
mesodermal  origin?  By  following  the  invagination  by  which  the  retina  is  formed 
do  you  find  any  suggestion  of  an  explanation  of  the  fact  that  the  sensitive  portion 
of  the  retina  (rods  and  cones,  Fig.  175)  is  directed  away  from  the  light?  Refer  to 
some  work  on  the  embryology  of  the  vertebrates  for  more  complete  series  of  figures. 

114.  Differentiation  and  Reintegration  of  Parts. — The  chap- 
ter thus  far  should  have  given  the  student  a  good  appreciation 
of  the  degree  to  which  the  work  of  a  complex  organism  is  divided 
up  among  the  organs  and  of  the  differences  found  in  the  organs 
themselves  because  of  this.  We  all  know  that  this  differentia- 
tion makes  for  efficiency.  There  are  two  sides  to  this,  however. 


THE    GENERAL   ANIMAL   FUNCTIONS  85 

In  becoming  suited  to  doing  one  kind  of  work  the  cells  cease,  to 
be  able  to  do  other  kinds.  The  various  cells  thus  become  mutu- 
ally dependent.  Unless  they  can  be  brought  to  work  together 
suitably  there  will  not  be  efficiency.  Division  of  labor  is  only 
one-half  of  efficiency.  There  must  also  be  a  reintegration  of 
these  special  parts  into  a  higher  kind  of  oneness.  It  is  this  that 
determines  the  state  of  the  individual  made  up  of  these  special- 
ized parts.  There  are  three  phases  of  this  reintegration. 

a.  Physical  Reintegration. — This  is  done  by  the  cement  be- 
tween cells,  by  the   connective  tissues,  the  skin,  the  skeleton, 
and  similar  tissues.     When  the  fertilized  egg  first  divided,  this 
cement  held  the  cells  together.     It  is  only  in  this  way  that  a 
multicellular  animal  could  arise.     In  the  mature  body  the  bil- 
lions of  cells  are  further  bound  together  by  these  connecting  tis- 
sues into  a  physical  unity  that  makes  it  possible  that  they  work 
together. 

b.  Nutritive  Reintegration. — This  physical  bond  would  mean 
little  but  for  a  deeper  one.     In  the  differentiation  of  the  cells , 
while  all  must  have  food  and  oxygen  and  all  produce  wastes 
which  they  must  get  rid  of,  the  various  cells  of  the  body  do  differ- 
ent things.     They  have  somewhat  different  needs  and  produce 
different  kinds  of  waste  products.     Some  cells  absorb  the  foods 
for  all;  others  absorb  oxygen.     Some  eliminate  carbon  dioxid 
from  the  system;  others  eliminate  the  urea  and  other  wastes 
for  all  the  cells.     It  follows  therefore  that  all  these  cells  must  be 
connected  in  a  nutritive  way.     This  is  done  by  the  body  fluids, 
including  chiefly  the  blood  and  lymph. 

In  addition  to  the  work  of  lungs,  digestive  epithelium,  the 
skin  and  kidneys  referred  to  above,  with  which  all  the  active 
cells  of  the  body  must  be  connected  in  this  chemical  interchange 
—the  liver,  the  spleen,  the  sex  glands,  the  thyroid,  the  adrenals 
and  other  groups  of  cells  take  from  or  add  to  the  blood  sub- 
stances that  vitally  change  it.  Very  often  these  special  sub- 
stances poured  into  the  blood  by  particular  organs,  as  the  testes 
or  the  thyroid,  will  be  carried  all  over  the  body  by  the  blood  and 
will  produce  profound  effects  on  the  well-being  of  other  groups  of 
cells  far  removed  from  them.  For  example,  it  seems  to  be  a 
product  of  the  sex  glands  which  gets  into  the  blood  and  produces 


86  ZOOLOGY 

growth  in  the  voice  box  and  vocal  cords  in  man.  If  the  thyroid 
is  diseased  or  removed,  so  that  its  product  is  no  longer  poured 
into  the  blood,  profound  changes  take  place  in  certain  other 
structures  in  wholly  different  parts  of  the  body.  If  the  healthy 
thyroid  tissue  of  another  individual  is  grafted  at  any  point  in 
the  body  of  such  a  diseased  individual  the  latter  will  become 
normal  again.  These  organs  are  believed  to  secrete  substances 
(hormones)  into  the  blood  which  control  activity  elsewhere. 

Indeed  we  are  coming  more  and  more  to  understand  that 
every  cell  in  the  body  changes  the  blood  in  some  degree  for  all 
the  other  cells  of  the  body.  It  takes  from  the  blood  as  it  passes 
and  at  the  same  time  pours  out  into  the  blood  the  by-products 
of  its  own  living.  We  must  think  of  the  blood  as  more  than 
just  a  means  of  connecting  lungs  and  intestine  and  kidneys  and 
the  general  system.  The  life  of  every  cell  in  the  body  is  con- 
ditioned by  the  make-up  of  this  medium  whose  composition  is 
determined  by  every  cell  in  the  body.  We  are  just  beginning  to 
appreciate  its  full  meaning;  but  this  chemical  integration  of  the 
body  is  profoundly  important. 

c.  The  Coordinating  Reintegration. — In  addition  to  the  phys- 
ical and  chemical  unity,  the  parts  of  the  body  must  work  in 
harmony.  When  food  is  eaten  the  digestive  glands  must  be- 
come active.  When  the  body  exercises  the  breathing  must 
become  more  rapid.  When  an  organ  acts  it  must  have  an  in- 
creased blood  supply.  When  we  undertake  to  walk,  scores  of 
muscles  must  contract  just  enough  and  in  the  right  relation  to 
other  muscles.  When  external  conditions  operate  we  must  be 
able  to  do  the  right  thing.  This  harmony  of  operation  among 
the  parts  is  coordination.  Harmony  between  the  action  of  the 
organism  and  the  outside  conditions  is  adaptation.  The  nervous 
system  reintegrates  these  numerous  diverse  units  of  which  we  are 
made,  and  makes  them  work  together.  All  the  active  cells  of 
the  body  are  connected  by  way  of  the  nervous  elements.  A  part 
of  the  work  of  the  hormones  is  also  of  this  higher  coordinating 
value.  It  has  been  found  by  experiment  that  the  pancreas,  for 
example,  is  brought  to  secrete  at  the  proper  time  by  substances 
in  the  blood  absorbed  from  the  digestive  tract  rather  than 
through  nervous  action  purely,  as  we  formerly  thought. 


THE  GENERAL  ANIMAL  FUNCTIONS  87 

115.  Summary. 

1.  Division  of  labor  and  differentiation  of  structure  proceed 
together   as  the  individual   develops.     All  tissues  retain   the 
power  of  using  food,  of  oxidation,  of  eliminating  useless  prod- 
ucts.    Other  functions  incidental  to  these  may  be  relegated  to 
special  cells  or  tissues. 

2.  Associations  of  tissues  to  accomplish  a  more  or  less  definite 
work  are  called  organs.     Organs  of  a  similar  kind  are  collectively 
known  as  systems  of  organs. 

3.  The  principal  functions  of  animals  and  the  organs  or 
systems  performing  the  work  may  be  classed  as  follows: 

Function  System 

(a)  Metabolism Nutritive. 

(6)  Support  and  protection. . .  Skeletal,  and  integumentary. 

(c)  Growth 

(d)  Reproduction Reproductive. 

(e)  Motion Muscular,    in   connection   with 

skeletal. 

(/)    Sensation Nervous,  including  sensory  epi- 
thelium. 

4.  Metabolism  or  nutrition  embraces  the  following  processes: 
(a)  Ingestion  of  food  (including  oxygen), 

(6)   Digestion, 

(c)  Absorption  (from  the  digestive  tract,  and  at  every  other 
cell  wall), 

(d)  Circulation, 

(e)  Assimilation  =  anabolism, 

(/)   Dissimilation  =  katabolism  (including  respiration), 
(g)  Secretion  and  excretion  (of  waste  matter  including  carbon 
dioxid). 

The  processes  in  (a),  (b),  (c),  and  (e)  are  anabolic,  i.e.,  add 
to  the  resources  of  the  body.  Those  in  (/)  and  (g)  are  katabolic, 
i.e.,  tend  to  destroy  the  materials,  develop  energy,  and  eliminate 
waste.  Circulation  contributes  to  the  accomplishment  of  both 
purposes. 

5.  The  supportive  skeletal  structures  may  be  internal,  or 
external,  or  both.     They  may  arise  as  a  secretion  of  the  super- 


88  ZOOLOGY 

ficial  cells  of  the  body,  or  consist  of  a  mixture  of  cells  and  in- 
ter-cellular substance.  Their  nature  and  arrangement  are 
profoundly  important  in  determining  the  distribution  of  the 
other  more  active  organs. 

6.  Growth  and  reproduction  are  the  outcome  of  the  nutri- 
tive processes.     Growth  is  increase  in  mass;  reproduction  is  the 
production  of  new  individuals  from  old.     Reproduction  always 
involves  cell  division  and  may  be  asexual  or  sexual.     The  latter 
normally  involves  two  parents.     In  it  two  cells,  which  may  be 
either  similar  or  dissimilar,  must  unite  before  development  will 
proceed. 

7.  The  nervous  and  muscular  systems  are  closely  related 
in  function.     Their  united  work  is  to  receive,  coordinate  and 
respond  to  the  external  or  internal  stimuli  affecting  the  animal. 
The  structures  to  receive  stimuli  (end  organs)  are  largely  super- 
ficial; the  coordinating  and  controlling  parts  (central  organ)  are 
deep-seated,  thereby  securing  protection  and  a  central  position; 
the  muscular  system  must  have  definite  relations  with  the  hard 
parts  upon  which  it  acts.     Thus  arises  the  necessity  of  connec- 
tives or  nerves  between  the  various  portions. 

8.  The  sense  organs  represent  areas  of  the  epithelium  which 
are  peculiarly  adapted  to  the  reception  of  some  one  of  the  forms 
of  stimulus  to  which  animals  are  subject,  supplemented  by  a 
more  or  less  complex  apparatus  which  serves  to  intensify  or 
modify  the  original  stimulus.     The  sense  organ  determines  the 
kind  of  stimulus  which  may  be  received,  but  the  central  nervous 
organ  determines  the  nature  of  the  sensation  which  results,  and 
the  response. 

9.  If  cells  and  organs  become  differentiated,  and  specialized 
in  the  division  of  labor,  it  becomes  doubly  essential  that  they  be 
brought  together  again  in  a  complete  unity  if  there  is  to  be  an 
efficient  individual.     This  reintegration  of  diverse  parts  is  (i) 
physical,  by  means  of  connective  and  supportive  tissues;  (2) 
chemical  and  nutritive,  by  means  of  blood  and  lymph;  and 
(3)  cooperative,  by  means  of  the  nervous  system. 

10.  Organs  with  different  origin  which  by  reason  of  similar 
function  have  come  to  look  alike  are  said   to  be  analogous. 
Organs  with  similar  origin  and  structure,  even  though  they  may 


THE   GENERAL   ANIMAL   FUNCTIONS  89 

appear    differently    because    of    their    differing    functions,    are 
homologous. 

1 1 6.  Topics  for  Investigation,  in  Library  and  Field. 

1.  What  are  the  advantages  and  the  disadvantages  of  divi- 
sion of  labor  and  differentiation?     Illustrate  your  views  very 
fully. 

2.  Illustrate  the  variety  of  foods  used  by  different  animals 
with  which  you  are  acquainted.     Classify  the  animals  you  know 
on  the  basis  of  their  food  preferences. 

3.  Compare  the  ways  in  which  animals  known  to  you  cap- 
ture  and   prepare   their   food   for   swallowing.     What   special 
structures  arise  in  connection  with  this  function  ? 

4.  Do  animals  have  any  power  of  storing  food  within  the 
body  for  future  use  ?     Compare  with  plants. 

5.  Compare  gills  and  lungs  as  to  general  form  and  arrange- 
ment and  see  in  what  ways  they  appear  to  you  to  be  suited  to 
their  particular  media,  i.e.,  gills  to  water  and  lungs  to  the  air. 
Why  might  not  the  conditions  be  reversed  ? 

6.  What  seem  to  you  to  be  the  comparative  advantages  and 
disadvantages  of  the  exoskeleton  and  endoskeleton. 

7.  Devices  to  accomplish  locomotion  in  animals  known  to  the 
student.     Find  as  many  variations  as  possible. 

8.  Select  four  animals,  as  diverse  as  possible,  representing 
each  of  the  following  conditions  of  locomotion : — through  the  air, 
through  the  water,  on  the  earth,  and  through  the  soil.     Compare 
the  problems  which  each  must  solve,  and  the  organs  by  which 
the  work  is  accomplished. 

9.  Compare  known  animals  as  to  rate  of  locomotion.     Do 
you  find  a  satisfactory  explanation  in  any  case  ? 

10.  Let  the  student  attempt  to  prove  that  the  dog  experi- 
ences the  same  sensations  which  we  have.     Hold  him  rigidly  to 
his  evidence. 

11.  Report  on  the  general  differences  between  the  eyes  of 
insects  and  of  vertebrates,  with  a  statement  of  their  structure 
and  the  work  done  by  each. 

12.  In  what  way  could  the  statocysts  possibly  act  as  organs 


go  ZOOLOGY 

to  enable  the  animal  to  appreciate  its  position  in  space,  and  thus 
maintain  its  equilibrium  ? 

13.  What  are  the  simpler  facts  connected  with  the  process 
of  absorption  or  osmosis  of  dissolved  substances  in  the  body  ? 

14.  Find  in  text-books  of  chemistry  a  fuller  account  of  the 
process  of  oxidation  and  why  it  results  in  a  liberation  of  energy. 

15.  Demonstrate  how  a  biconvex  lens  forms  an  image  of 
objects.     Why  inverted  ? 


CHAPTER  VII 

PROMORPHOLOGY 

117.  We  have  seen  in  the  preceding  chapters  how  the  work  which  an  organism 
must  do  is  divided  among  its  parts,  and  that  the  parts  become  specialized  in  con- 
nection with  this  division  of  labor.     This  complexity  which  is  known  as  organiza- 
tion is,  in  any  animal,  the  result  of  forces  both  within  and  without  the  animal,  and 
expresses  the  adaptation  of  the  internal  structures  to  each  other  and  to  external 
conditions.     The  simplest  organism  known  is  thus  organized.     Organization  is 
merely  more  evident  in  the  more  complex  organism.     In  addition  to  the  fact  of 
the  organization  and  heterogeneity  of  structure  it  is  easy  to  see,  after  examining  a 
number  of  animals,  that  these  different  parts  are  not  thrown  together  without 
some  definite  order.     For  example,  the  ordinary  vertebrates  move  with  their  long 
axis  horizontal,  and  possess  certain  organs  that  we  always  expect  to  see  at  the 
anterior  end;  their  appendages  are  arranged  in  a  definite  way  in  relation  to  the 
long  axis.     The  parts  of  the  starfish  are  arranged  according  to  a  different  but 
equally  definite  plan.     All  consideration  of  the  general  plan  according  to  which 
animals  are  constructed  may  be  called  Promorphology.     The  fundamental  plan 
may  be  similar  in  groups  of  animals  which  are  otherwise  very  different,  because  of 
similar  external  conditions  and  similar  modes  of  life. 

118.  Definition  of  Sections. — In  trying  to  express  the  plan  of  structure  in 
animals  it  is  convenient  to  have  in  mind  certain  planes  to  which  we  can  refer  the 
parts.     A  section  perpendicular  to  the  main  axis  of  an  organism  or  of  an  organ  is 
called  a  transverse  or  cross  section.     The  longitudinal  median  section  separating 
the  body  into  right  and  left  halves  is  a  sagittal  section.     A  longitudinal  section, 
perpendicular  to  the  sagittal  and  separating  the  dorsal  (or  back)  and  ventral  (or 
belly)  portions  of  the  body  is  described  as  a  frontal  section.     An  animal  is  said  to 
be  symmetrical  with  regard  to  any  of  these  planes  when  the  parts  severed  by  the 
plane  are  essentially  similar. 

119.  Axiality. — As  an  organism  grows  from  its  small  beginnings  in  the  fertilized 
ovum,  or  from  a  spore  in  the  simpler  forms,  the  new  materials  may  be  added  more 
or  less  uniformly  so  that  a  mere  increase  in  size  results;  or  growth  may  take  place 
more  rapidly  along  some  radii  than  along  others,  making  it  depart  from  its  original 
spherical  form ;  or  materials  or  organs  of  one  kind  may  occur  along  one  radius  and 
different  ones  on  another  (as  in  Fig.  48).     These  lines  of  special  growth  and  develop- 
ment are  called  axes.    We  may  investigate  them  as  to  their  number,  their  space- 
relations  to  each  other,  and  the  likeness  or  unlikeness  of  the  two  ends  or  poles  of 
each  axis. 

120.  Types  of  Symmetry. — It  is  desirable  to  distinguish  the  following  types: 
i.  In  a  spherical  organism  in  which  no  differentiation  is  apparent  (as  in  a 

simple  spherical  cell,  or  blastula,  Figs.  13  A,  3;  46)  any  plane  passing  through  the 
centre  divides  it  into  symmetrical  parts  and  all  the  axes  are  essentially  equal 

91 


92  ZOOLOGY 

In  such  a  case  there  may  be  said  to  be  an  infinite  number  of  similar  axes,  and  the 
poles  of  each  axis  are  similar.     This  may  be  described  as  universal  symmetry. 


FIG.  46. 


FIG.  47. 


FIG.  46.     Spherical  cell   (resting  stage  of  Amoeba)   illustrating  general  or  universal  symmetry. 
Any  plane  passing  through  the  centre  will  divide  it  into  two  essentially  equal  portions. 

Question  on  the  figure. — What  prevents  this  animal  being  a  perfect  illustration 
of  universal  symmetry? 

FIG.  47.     Amoeba  in  active  condition.     Entirely  unsymmetrical. 


FIG.  48. 


FIG.  48.  Aclinomma  aster acanthion,  a  Radiolarian  with  a  limited  number  of  specialized  radii 
(axes),  symmetrically  arranged  about  the  centre.  A,  whole  animal  with  portion  of  two  spheres  of 
shell  removed.  B,  section,  showing  relation  of  the  protoplasm  to  the  skeleton,  n,  nucleus;  p, 
protoplasm;  sk.,  skeleton.  From  Parker  and  Haswell. 

Questions  on  the  figures. — In  what  way  does  this  species  differ  in  symmetry 
from  Fig.  46?  How  many  specially  developed  axes  appear  to  be  present?  By 
how  many  planes  may  the  organism  be  divided  into  essentially  equal  portions? 

2.  An  organism  may  be  wholly  asymmetrical,  without  any  definite  form,  the 
axes  being  without  regular  arrangement.  (Amoeba  in  its  active  stages,  Fig.  47, 
some  Sponges.)  In  other  instances  the  form  may  be  definite  and  axes  developed; 


PROMORPHOLOGY  93 

but  the  structure  is  such  that  no  plane  will  separate  the  animal  into  symmetrical 
parts  (Paramecium  and  many  other  active  Protozoa;  see  Figs.  68-71). 

3.  As  a  variation  of  the  universally  symmetrical  condition  seen  in  I,  a  limited 
number  of  axes  may  become  distinguishable  from  the  others  by  some  specialization 
of  structure  (Fig.  48).     These  special  axes  are  similar  and  their  two  poles  are  alike. 

4.  Starting  again  from  the  undifferentiated  spherical  form,  one  of  its  numerous 
similar  axes  may  come  to  differ  from  all  those  perpendicular  to  it  by  increased  or 
diminished  length,  or  by  a  difference  in  construction.     This  special  axis  is  to  be 
known  as  the  principal  axis.     The  poles  of  the  principal  axis  do  not  usually  remain 
alike.     Perpendicular  to  this  principal  axis  one  may  select  an  indefinite  number  of 
subordinate  axes  which  are  essentially  similar  to  one  another.     The  poles  of  each 
subordinate  axis  are  alike.     Such  a  condition  is  realized  in  the  simplest  gastrulae 
(Fig.  13;  A  4,  B  3,  C  2).     Any  plane  including  the  principal  axis  will  divide  such 

FIG. 

R 

ab.  o. 


o 
b1 

FIG.  49.  Diagram  of  Medusa,  illustrating  radial  symmetry.  A,  viewed  from  the  oral  end  of 
the  principal  axis;  B,  a  section  along  the  principal  axis  and  through  one  of  the  subordinate  axes  ao1: 
o,  ab.  o,  the  oral  and  aboral  poles  of  the  principal  axis;  a,  a1,  and  b,  b1,  the  similar  poles  of  the  two 
chief  subordinate  axes. 

Questions  on  the  figures. — Are  the  poles  of  the  oral-aboral  axis  alike  or  unlike? 
How  many  clearly  differentiated  secondary  axes  are  there?  What  would  be  the 
appearance  of  a  section  midway  between  aal  and  bbl?  Would  the  resulting  halves 
be  symmetrical?  Compare  this  condition  with  the  definition  of  radial  symmetry 
in  the  text.  Find  other  illustrations  of  radial  symmetry  in  the  figures  of  this 
book. 

an  organism  into  two  equal  halves.  In  general  external  appearance  a  hen's  egg 
would  illustrate  the  type.  This  is  the  least  differentiated  form  of  what  is  known  as 
radial  symmetry. 

Two  important  variations  from  this  simple  condition  of  radial  symmetry  are 
found  in  the  animal  kingdom: 

(a)  Special  organs,  such  as  those  of  locomotion  and  the  like,  may  be  developed 
about  the  principal  axis.  These  usually  come  to  be  arranged  in  a  limited  number 
of  the  planes  which  may  be  passed  through  the  principal  axis.  Considered  from 
the  point  of  view  of  the  subordinate  axes  this  means  that  there  are  a  limited  number 
of  special  axes  (Fig.  49,  aal  and  bbl)  perpendicular  to  the  principal  axis  (Fig.  49, 


94 


ZOOLOGY 


o-ab.o)  instead  of  an  indefinite  number  as  in  the  former  case.  These  special  sub- 
ordinate axes  are  usually  3,  4,  or  5,  or  some  multiple  of  these  numbers.  The  num- 
ber however  may  be  reduced  to  two  in  which  the  four  poles  are  all  alike.  Many  of 
the  medusae,  coral  polyps,  and  some  echinoderms  illustrate  this  type  of  symmetry. 
(b)  A  further  variation  of  (a)  is  seen  in  the  fact  that  in  some  animals,  otherwise 
similar  to  those  described  in  (a),  one  of  these  special  axes  perpendicular  to  the 
principal  axis  comes  to  differ  from  the  other.  The  two  poles  of  each  of  the  sub- 
ordinate axes  are  essentially  alike,  but  are  unlike  the  poles  of  the  other  subordinate 

FIG.  50. 


ab.  o. 

Pic.  50.  Diagram  of  the  sea-anemone,  illustrating  another  type  of  radial  symmetry.  A, 
cross  section;  B,  longitudinal  section.  Lettering  as  in  Fig.  49.  c,  the  chamber  between  the  mes- 
enteries (m). 

Questions  on  the  figures. — How  does  the  symmetry  of  the  anemone  compare 
with  that  of  the  Medusa?  Express  the  difference  clearly  in  terms  of  the  axes  and 
their  poles.  Are  aal  and  bbl  strictly  similar  axes?  Do  their  planes  divide  the 
animal  into  halves?  Are  the  four  halves  thus  obtained  equivalent?  In  B  what 
difference  in  the  position  of  the  section  will  account  for  the  differences  on  the  right 
and  left  side  of  the  figure? 


axis.  This  arrangement  is  found  in  the  sea-anemone  (Fig.  50).  The  differences 
between  aal  and  bb l  are  not  usually  so  great  that  we  cease  to  speak  of  the  form  as 
radially  symmetrical.  It  is  of  importance  to  know  that  in  the  radially  symmetri- 
cal animals  the  principal  axis,  whether  longer  or  shorter  than  the  subordinate 
axes,  is  normally  perpendicular  to  the  substratum  on  which  the  animal  rests,  or  to 
which  it  is  attached. 

5.  If  such  an  animal  as  was  last  described  were  to  have  its  principal  axis 
horizontally  placed,  with  one  of  its  two  subordinate  axes  vertical  and  the  other 


PROMORPHOLOGY 


95 


horizontal,  and  were  maintained  in  this  position,  it  would  likely  happen  that  the 
formerly  similar  poles  of  the  new  vertical  axis  would  become  unlike,  because 
subjected  to  different  influences.  These  poles  are  known  as  the  dorsal  and  ventral 
poles.  The  poles  (right  and  left)  of  the  other  transverse  or  subordinate  axis  would 
remain  similar,  as  they  are  subjected  in  the  long  run  to  similar  conditions.  This 
gives  us  the  condition,  found  in  all  the  higher,  free-moving  animals,  known  as 
bilateral  symmetry.  It  consists,  to  recapitulate,  of  (i)  a  main  axis  (antero-posterior 

FIG.  51. 


FIG.  51.  Diagram  of  the  cross  section  of  a  fish,  showing  the  bilateral  symmetry  of  the  parts: 
dv,  dorsoventral  axis;  rl,  right-left  axis.  a. p.,  anterior  appendage;  b.c.,  body  cavity;  ch,  notochord; 
d.f.,  dorsal  fin;  g,  gut;  h,  heart;  A. a.,  haemal  arch;  m,  muscles;  n.a.,  neural  arch;  sp,  spinal  cord;  t.c- 
vertebral  column. 

Questions  on  the  figure. — In  what  respects  is  the  symmetry  as  shown  in  this 
cross  section  different  from  that  shown  in  the  cross  section  of  sea-anemone?  Com- 
pare carefully,  and  express  your  conclusions  in  terms  of  the  axes  and  their  poles. 
Find  other  figures  in  this  book  illustrating  bilateral  symmetry. 

axis)  usually  horizontal  and  with  dissimilar  poles;  (2)  a  transverse  axis,  usually 
vertical  (dor so-ventral  axis)  with  dissimilar  poles;  and  (3)  a  transverse  axis  perpen- 
dicular to  the  other  two,  horizontally  placed,  with  poles  alike  (right-left  axis). 
Such  an  animal  has  only  one  plane  (the  sagittal)  by  which  it  may  be  divided  into 
symmetrical  halves  (Figs.  51,  31,  101). 


96  ZOOLOGY 

121.  Antimeres. — There  is  a  striking  tendency  among  organisms  to  repeat  or 
duplicate  organs  or  parts.     This  we  have  seen  in  the  occurrence  of  similar  rays 
about  the  main  axis,  in  radially  symmetrical  animals  like  the  starfish.     Parts  thus 
repeated  are  known  as  antimeres.     The  term  is  also  applied  to  the  right  and  left  or 
paired  halves  of  bilaterally  symmetrical  animals. 

122.  Metameres. — When  the  parts  or  organs  are  repeated  in  a  linear  sequence 
along  the  main  axis,  as  in  the  segments  or  rings  of  the  Earthworm,  the  arrangement 
is  described  as  segmental  or  metameric.     Metamerism  may  be  shown  both  by  the 
internal  and  external  structures.     The  vast  majority  of  the  elongated,  bilaterally 
symmetrical  animals  are  segmented.     In  the  higher  Vertebrates  it  is  not  manifest 
externally,  but  is  shown  in  the  vertebrae,  the  nerves,  etc. 

123.  Appendages. — Nearly  all  the  animals,  whatever  the  fundamental  sym- 
metry may  be,  have  appendages  of  one  kind  or  another  for  locomotion,  capture  of 
food,  protection,  respiration,  and  the  like.     These  outgrowths  from  the  body  may 
be  generally  distributed  over  the  body  surface  (as  cilia  in  some  Protozoa  and  free- 
swimming  larvae) ;  or  radially  arranged, — often  about  the  mouth,  as  in  many  radi- 
ally symmetrical  animals  (Figs.  49,  50,  81) ;  or  in  a  right  and  left  series  in  bilaterally 
symmetrical  animals  (Figs.  131,  137,  191).     The  paired  appendages  of  bilateral 
animals  may  be  attached  dorsally,  laterally,  or  ventrally,  as  determined  by  the 
uses  they  serve.     They  may  be  uniformly  distributed  along  the  axis,  one  or  more 
pairs  to  each  metamere,  as  in  some  arthropods,  or  be  confined  to  special  segments  in 
certain  regions  of  the  body,  as  in  the  higher  arthropods  and  the  vertebrates. 

124.  Practical  Exercises. — Let  the  student  find  illustrations,  from  the  animals 
with  which  he  is  acquainted,  of  paired  appendages  with  dorsal  attachment;  with 
lateral;  with  ventral.     What  is  the  work  to  be  done  by  each?     Does  their  position 
appear  to  be  of  advantage  in  the  performance  of  it?     Find  likewise  animals  in 
which  the  appendages  are  clustered  at  the  anterior  portion  of  the  body;  some  in 
which  only  posterior  appendages  are  found,  or  at  least  are  better  developed  than 
the  anterior.     Does  the  arrangement  seem  in  any  way  related  to  the  habits  and 
surroundings  of  the  animal? 

125.  Specialization   of   Metameres. — In  the  lowest  segmented  animals,   as 
worms,  the  metameres  are  much  alike  in  external  form,  in  their  appendages,  and 
in  the  contained  structures.     In  the  adult  insects  this  becomes  less  true,  and  the 
various  segments  are  specialized  for  particular  duties.     The  segments  in  the  head 
region  become  very  different  from  the  body  segments.     The  same  is  even  more 
true  of  the  higher  vertebrates.     This  progressive  differentiation  of  a  distinct  head 
is  one  of  the  most  remarkable  facts  to  be  noted  in  animal  development.     Accom- 
panying the  specialization  of  groups  of  segments  in  various  parts  of  the  body  we 
often  see  the  complete  fusion  of  such  similar  segments  for  the  better  performance 
of  their  common  work  (as  in  the  head  of  insects  and  vertebrates  and  the  thorax  in 
insects  and  Crustacea). 

126.  Formation  of  New  Segments  and  Regeneration. — In  many  of  the  animals 
in  which  the  segments  seem  of  nearly  equal  value  there  is  a  more  or  less  continuous 
formation  of  new  segments.     By  this  process  the  organism  increases  in  length  and 
in  the  number  of  its  segments,  and  frequently,  with  the  aid  of  a  somewhat  similar 
process,  produces  two  individuals  by  division,  as  in  many  worms.     Such  a  pro- 
ceeding necessitates  the  formation  of  a  new  head  or  tail  in  each  of  the  daughters, 


PROMORPHOLOGY  97 

by  a  segment  which,  in  the  mother,  was  a  body  segment.  When  such  an  animal 
is  artificially  cut  in  two,  each  half  may  reproduce  segments  like  those  which  have 
been  removed  from  it.  This  is  known  as  regeneration.  Naturally  this  ceases  to  be 
possible  in  animals  in  which  the  segments  become  more  highly  specialized;  yet 
even  in  the  highest  animals  some  power  of  replacing  lost  tissue  or  even  lost  organs 
remains  (as  in  healing  of  wounds,  formation  of  a  new  tail  by  lizards,  etc.).  It  is 
recognized  as  a  general  law  that  in  making  these  repairs  or  healings  the  newly 
formed  material  tends  to  restore  the  symmetry  possessed  at  the  outset. 

127.  Summary. — 

1 .  Promorphology  treats  of  the  ground  plan  in  accordance  with  which  the  parts 
of  animals  are  combined. 

2.  Symmetry  relates  to  the  possibility  of  passing  one  or  more  planes  through 
the  animal  and  obtaining  similar  portions  on  either  side  of  these  planes. 

3.  If  no  such  division  is  possible  the  organism  is  described  as  without  symmetry. 
If  each  of  three  mutually  perpendicular  planes  separates  the  animal  into  equivalent 
portions,  it  may  be  described  as  universally  symmetrical.     If  no  plane  transverse 
to  the  main  axis  can  divide  the  animal  into  symmetrical  parts,  and  two  or  more, 
which  split  the  animal  along  the  main  axis,  are  capable  of  doing  so,  we  describe  the 
form  as  radially  symmetrical.     When  there  is  only  one  such  plane  capable  of 
separating  the  body  into  equal  parts  we  have  the  condition  of  bilateral  symmetry, 
which  represents  the  highest  condition  of  development,  that  of  active  animals. 

4.  Antimeres  are  parts  of  animals  repeated  on  different  sides  (two  or  more)  of 
the  main  axis  of  the  body. 

5.  Metameres  are  parts  repeated  in  the  main  axis,  i.e.,  one  behind  another. 
The  successive  metameres  may  be  almost  entirely  alike  (homonomous) ,  or  they  may 
become  much  differentiated  in  the  performance  of  diverse  functions  (heteronomous). 

6.  From  the  main  trunk  of  animals  special  appendages  often  appear.     They 
usually  adapt  themselves  to,  and  accentuate,  the  fundamental  symmetry  of  the 
organism.     They  may  therefore  be  asymmetrically  placed,  or  uniformly  distributed 
over  the  entire  surface,  or  along  the  radii,  or  in  pairs  as  in  the  bilaterally  sym- 
metrical forms.     There  are  typically  one  or  more  pairs  to  each  metamere,  though 
this  number  may  be  much  reduced.     Paired  appendages,  in  series,  are  regarded  as 
homologous. 

7.  Many  animals  have  the  power  of  restoring  by  growth  parts  or  segments 
which  have  been  lost  (regeneration).     In  the  lower  segmented  forms  this  power 
is  closely  associated  with  the  power  of  increasing  the  number  of  new  segments  in 
an  uninjured  animal.     In  heteronomously  segmented  animals  both  these  powers 
are  less  manifest. 

128.  Topics  for  Investigation. 

1.  Determine  the  nature  and  degree  of  symmetry  in  (i)  the  sponge  of  com- 
merce; (2)  skeleton  of  starfish;  (3)  crayfish  or  grasshopper. 

2.  What  is  the  final  criterion  by  which  you  determine  which  is  the  anterior  and 
which  the  posterior  end  of  an  animal?     Justify. 

3.  Find  among  animals  of  your  acquaintance  instances  of  difference  between 
the  dorsal  and  ventral  surfaces  as  to  color,  form,  etc.,  and  see  if  you  can  discover 
any  possible  advantage  resulting  therefrom. 

4.  What  degree  of  difference  have  you  ever  noticed  between  the  right  and  left 
halves  of  the  body  in  various  animals.     Is  perfect  bilateral  symmetry  ever  found  ? 

7 


98  ZOOLOGY 

5.  Can  you  assign  any  reason  for  the  location  of  the  sense  organs  at  the  anterior 
end  of  the  body?     In  a  given  individual?     In  the  race  to  which  the  individual 
belongs?     (Distinguish  between  cause  and  advantage).     Do  you  think  they  occur 
here  because  it  is  anterior,  or  is  it  anterior  because  they  occur  here? 

6.  Why  are  vehicles  (made  by  man)  bilaterally  symmetrical?      t 

7.  Express  in  tabular  form  the  differences  between  the  poles  of  the  three  axes 
of  bilateral  animals,  and  what  you  can  gather  as  to  the  causes  of  the  differences. 

8.  For  what  kind  of  life  does  bilateral  symmetry  fit  the  animal? 

9.  For  what  kind  of  life  does  radial  symmetry  seem  suited  ?     Verify  by  illustra- 
tions. 

10.  To  what  extent  do  animals  seem  able  to  regenerate  lost  parts?     Trace 
out  the  conditions  in  various  groups,  as  far  as  your  reference  library  will  allow. 

11.  Is  there  any  apparent  limits  to  the  numbers  of  metameres  which  animals 
may  possess?     What  degree  of  constancy  or  variability  in  this  particular  can  you 
discover  in  various  individuals  of  the  same  species? 


CHAPTER  VIII 

DIFFERENTIATION  OF  INDIVIDUALS  AND  ADAPTATION 

129.  The  Individual  and  its  Environment. — We  have  thus 
far  considered  the  mature  individual  as  the  end  for  which  the 
various  developmental  processes  exist.     It  has  been  seen  that 
the  individual  becomes  complex  as  its  parts  grow  and  become 
differentiated  to  do  the  work  necessary  to  the  well-being  of  the 
animal.     In  this  differentiation  the  parts  become  dependent 
upon  each  other,  and  in  the  healthy  state  they  work  harmo- 
niously among  themselves.     It  is  now  necessary  to  pass  from  the 
consideration  of  these  internal  structures  and  relations  in  order 
to  consider  the  individual  animal  as  a  unit  in  its  relations  to 
everything  about  it,  that  is,  to  its  environment.     This  term 
includes  not  merely  the  inanimate  materials  and  conditions 
surrounding  an  organism,  but  in  addition  all  living  things  both 
plant  and  animal  which  directly  or  indirectly  influence  it.     The 
environment  of  no  two  animals  is  the  same,  nor  is  it  the  same  for 
any  given  animal  for  two  moments  in  succession.     This  con- 
tinual change  in  the  environment  leaves  its  impress  on  the  struc- 
ture and  habits  of  all  organisms.     Every  individual  is  thus  re- 
lated to  its  own  environment  from  day  to  day;  in  addition  to 
this,  all  the  individuals  of  any  generation,  owing  to  the  facts  of 
reproduction,  are  also  to  be  considered  in  the  light  of  the  con- 
ditions to  which  their  parents  have  been  subjected  from  the 
remotest  time.     The  study  of  the  individual  in  its  relations  to 
its  environment  brings  the  student  face  to  face  with  many  very 
important    problems.     No    department    of    zoology    is    more 
interesting. 

130.  Heredity. — One  does  not  study  organisms  very  long 
without  being  impressed  with  two  things:  first,  that  there  are 
remarkable  similarities  among  them,  even  among  those  little 
related ;  and  second,  that  there  are  interesting  differences  among 

99 


100  ZOOLOGY 

even  those  whose  kinship  would  entitle  them  to  great  likeness. 
There  is  always  a  disposition  among  students  to  feel  that  the 
likenesses  are  due  to  internal  causes  and  that  the  unlikenesses  are 
due  in  some  way  to  the  varying  external  influences.  In  other 
words  the  former  are  thought  to  be  due  to  heredity  and  the  latter 
to  the  environment. 

Characteristics  which  animals  receive  from  the  union  of  the 
germ  cells  are  described  as  hereditary.  We  ascribe  the  fact  that 
the  fertilized  hen 's  egg  produces  a  fowl  and  a  frog's  egg  a  frog  to 
the  action  of  heredity.  No  less  is  the  repetition  in  the  child  of 
minute  parental  peculiarities  of  feature  and  form  a  fact  of 
inheritance.  While  these  likenesses  are  due  to  the  action  of  the 
internal  forces  of  heredity ,  it  must  not  be  deemed  that  heredity 
is  a  purely  conservative  influence  in  the  life  of  the  organism. 
The  offspring  of  two  parents  may  inherit  qualities  entirely 
different  from  their  parents  and  thus  present  differences 
among  themselves  due  solely  to  inheritance.  The  offspring 
may  also  present  such  a  mingling  of  the  qualities  inherited 
from  their  ancestors  as  to  possess  characteristics  decidedly 
new  to  any  of  them.  Thus  likeness  to  parents  and  unlike- 
ness  to  parents  may  equally  be  due  to  heredity.  It  at  once 
perpetuates  old  qualities  and  introduces  variations. 

It  was  formerly  considered  that  all  the  characteristics  which 
parents  possess  are  equally  subject  to  inheritance,  but  it  is  now 
denied  by  many  biologists  that  the  qualities  which  a  parent 
acquires  in  its  own  lifetime,  as  the  result  of  its  own  actions  or  of 
the  environment,  are  capable  of  being  transmitted.  In  the 
light  of  what  was  said  about  the  parallel  development  of  prim- 
ordial germ  cells  and  body  cells  (§49),  this  is  just  another  way 
of  saying  that  changes  that  come  to  the  body  cells  may  not  spe- 
cifically change  the  germ  cells  so  that  they  will  later  rise  to  the 
same  sort  of  a  change  in  the  body  cells  which  arise  from  them. 
It  is  unquestioned,  however,  that  qualities  received  by  inheritance 
from  the  preceding  generations  are,  under  favoring  circum- 
stances, capable  of  being  transmitted  to  the  following. 

131.  The  Bearers  of  Heredity. — It  follows  from  the  fact 
that  the  adult  organism  is  produced  from  the  union  of  the  male 


DIFFERENTIATION    OF   INDIVIDUALS    AND    ADAPTATION  IOI 

and  female  elements  that  these  two  cells  are  in  some  way  en- 
dowed to  carry  the  parental  qualities.  They  are  all  that  pass 
from  the  old  generation  to  the  new.  There  are  strong  evidences 
that  the  chromatic  elements,  or  chromosomes,  in  the  nuclei  of 
the  male  and  female  cells  are  the  most  important  material  struc- 
tures by  means  of  which  transmission  is  effected.  The  chromo- 
somes of  the  fertilized  ovum  are  contributed  equally  by  the  male 
and  female  elements,  and  they  are  the  only  structures  in  the 
sperm  and  ovum  which  are  apparently  equal  in  amount.  This 
taken  in  connection  with  the  fact  that  one  parent  does  not  seem 
to  have  any  more  power  than  the  other,  on  the  average,  to  influ- 
ence offspring,  furnishes  a  basis  for  the  belief  that  the  chromo- 
somes are  the  physical  basis  of  heredity. 

Many  good  students  of  the  subject  believe  that  the  cytoplasm 
is,  equally  with  the  chromosomes,  a  carrier  of  heredity  qualities. 
They  think  that  cytoplasm  is  responsible  for  those  qualities 
that  both  sexes  have  in  common.  That  is,  the  cytoplasm  con- 
serves the  character  of  the  species.  The  chromosomes  are  held 
to  be  more  responsible  for  those  qualities  in  which  the  individuals 
of  a  species  differ.  Recent  investigations  seem  to  show  that 
the  male  and  female  chromosomes  retain  their  distinctness  in 
large  measure  and  are  equally  distributed  to  all  the  nuclei  of 
the  developing  embryo. 

132.  Library  Exercises. — The  student  may  increase  his  knowledge  of  the  facts 
of  heredity  by  endeavoring  to  find  answers  to  the  following  questions.  What  is 
atavism,  and  what  explanations  have  been  offered  for  it?  Do  the  male  and  female 
seem,  as  a  rule,  to  have  equal  power  of  transmitting  their  individual  characteristics? 
Cite  some  facts  tending  to  show  that  the  nucleus  is  especially  concerned  in  trans- 
mitting parental  qualities;  that  the  chromosomes  are  instrumental  therein.  What 
are  the  essential  features  of  the  old  "  perf ormation  "  hypothesis  to  account  for  the 
fact  that  an  adult  similar  to  the  parent  springs  from  an  egg?  Examine  some  of 
the  principal  theories  of  inheritance:  Darwin's  "pangenesis;"  Brooks'  modifica- 
tion of  it;  Weismann's  "continuity  of  germ-plasm,"  etc.  What  is  Mendel's  law 
of  inheritance? 

133.  Variability. — Notwithstanding  the  fundamental  like- 
ness existing  between  parent  and  offspring,  and  among  the  off- 
spring of  common  parents,  no  two  individuals  even  among  the 
lowest  animals  are  exactly  alike.  This  fact  of  variation  is  only 
less  fundamental  than  the  fact  of  likeness.  Variation  among 


102  ZOOLOGY 

animals  appears  to  depend  upon  two  sets  of  considerations: 
(i)  the  physical  and  chemical  instability  of  the  protoplasm  of 
which  animals  are  so  largely  composed,  and  (2)  the  diversity  of 
the  environment  in  the  broadest  sense.  Through  the  interaction 
of  these  two  influences,  even  if  all  individuals  were  alike  at  the 
start,  it  would  only  be  a  question  of  time  until  the  offspring 
derived  from  them  would  present  noteworthy  differences. 
Such  differences  would  tend  to  increase  with  the  lapse  of  time. 
This  is  the  more  true  in  proportion  to  the  degree  in  which  varia- 
tions are  capable  of  being  transmitted  under  the  influence  of 
heredity.  So  far  as  we  can  tell  organisms  may  vary  to  any 
amount  if  we  give  time  enough. 

134.  The  Part  Played  by  the  Environment  in  Producing 
Variation  while  not  completely  understood  must  be  recognized 
as  very  real.  Even  though  much  stress  must  be  put  upon  the 
hereditary  complexity  and  instability  of  protoplasm  as  the 
source  of  variations,  it  is  evident  that  the  external  conditions 
serve  as  stimuli  to  produce  the  changes  on  the  inside.  For 
example,  it  is  a  matter  of  common  observation  that  the  quantity 
and  quality  of  food  greatly  influence  not  merely  the  rate  of 
growth  of  the  body  but  the  size  and  quality  of  the  adult  organ- 
ism as  well.  Life  would  be  impossible  without  food,  oxygen, 
water  and  suitable  temperature.  Any  variation  in  these  con- 
ditions at  once  has  its  effect  upon  the  organism.  Experiment 
shows  that  the  varying  degrees  of  salinity  of  the  water  may  be 
accompanied  by  striking  individual  differences  of  form  in  certain 
marine  animals.  Caterpillars  of  certain  butterflies  placed  in 
boxes  lined  with  differently  colored  papers  develop  pupae  with 
colors  harmonizing  with  those  of  the  boxes  containing  them. 
Colors  in  various  animals  are  intensified  or  changed  by  special 
foods  or  by  changed  temperature.  In  general  it  may  be  said 
that  changes  in  any  of  the  conditions  important  to  animal  life 
produce  some  change  or  variation  in  the  body  of  those  animals 
subjected  thereto.  Since  this  is  true,  it  becomes  inevitable  that 
the  various  individual  animals  on  the  earth  are  differentiated 
from  each  other  somewhat  as  was  seen  to  be  the  case  with  the 
cells  and  tissue  of  which  the  individual  itself  is  composed.  The 


DIFFERENTIATION    OF   INDIVIDUALS    AND    ADAPTATION  103 

following  paragraphs  trace  out  some  of  the  ways  in  which  this 
differentiation  of  individuals  takes  place,  the  relations  of  the 
various  organisms  to  each  other  and  to  the  environment. 

135.  The  Struggle  for  Existence. — All  animals  (with  a  few 
possible  exceptions  in  those  which  possess  chlorophyll)  depend 
ultimately  upon  green  plants  for  food,  those  which  live  on  other 
animals  no  less  than  those  which  use  plant  food  directly.  Only 
a  limited  amount  of  vegetation  can  be  supported  by  the  earth 
without  cultivation.  The  number  of  animals  therefore  which 
can  find  a  livelihood  on  the  earth  is  in  turn  restricted.  There  is, 
however,  no  such  limit  of  the  powers  of  reproduction,  either 
among  plants  or  animals.  Any  pair  of  organisms  if  unchecked 
could  in  a  very  few  years  supply  descendants  enough  to  populate 
the  earth  up  to  its  full  powers  of  support.  That  they  do  not 
thus  multiply  at  a  geometric  ratio  is  due  solely  to  the  influences 
at  work  to  destroy  these  descendants.  Any  group  of  organisms 
will  hold  its  own  when,  on  an  average,  a  pair  of  individuals  can  in 
a  lifetime  bring  to  maturity  another  pair  to  take  their  place. 
More  than  this  means  conquest  of  new  territory;  less  than  this, 
the  extinction  of  the  group.  When  we  recall  that  all  organisms 
have  this  unlimited  power  of  reproduction,  it  is  easy  to  see  that  a 
time  must  soon  come  when  a  struggle  for  food  and  a  foothold  on 
the  earth  is  inevitable.  The  struggle  would  be  more  intense 
between  those  organisms  which  demand  the  same  kind  of  food, 
that  is,  among  kindred.  This  is  the  fundamental  struggle.  It 
would  be  complicated  by  the  fact  that  some  groups  of  animals 
prey  upon  others,  and  that  the  primary  conditions  of  life,  as 
water,  temperature,  etc.,  are  subject  to  striking  changes.  These 
facts  tend,  by  just  so  much  as  they  destroy  individuals,  to 
relieve  the  struggle  within  the  species,  and  to  introduce  new  fac- 
tors which  give  great  variety  and  interest  to  the  life  problems  of 
animals.  There  is  nothing  more  certain  than  that  this  struggle 
has  occupied  organisms  practically  from  the  beginning,  and  all 
our  explanations  of  present  conditions  must  take  note  of  the 
fact.  All  the  important  structures  and  activities  of  animals  are 
modified  by  this  competition  for  a  livelihood. 

136.  Library  Exercises. — The  student  should  be  invited  to  make  real  to  him- 
self the  possibilities  of  a  geometrical  increase  as  applied  to  organisms.     Take 


104  ZOOLOGY 

the  known  rate  of  increase  (that  is,  the  total  number  of  descendants  in  an 
average  lifetime)  of  a  number  of  common  animals  and  determine  the  possible 
living  descendants  in  a  specified  time.  Find  references  concerning  infusoria, 
insects,  fish,  man.  Have  you  any  observations  relating  to  the  reality  of  the 
struggle  for  food  among  animals? 

137.  Natural  Selection. — In  spite  of  this  power  of  reproduc- 
tion we  see  that,  on  the  average,  individuals  do  not  increase.     The 
earth  is  no  more  thickly  inhabited  by  animals  today  than  it  has 
been  for  countless  ages.     The  proportions  of,  different  animals 
vary  now  and  again,  but  that  is  all.     Out  of  a  family  of  one 
hundred  young  individuals  striving  for  a  foothold,  no  two  of 
which   are   alike,  ninety-eight  will  be  destroyed.     Which  will 
survive?     Barring  accidents  beyond  the  powers  of  any  of  the 
individuals  to  resist,  those  will  survive  which  possess  or  acquire 
some  quality,  structure,  or  habit,  suited  to  the  struggle  in  which 
they  find  themselves.     This  may  be  a  matter  of  strength,  of 
speed  in  eluding  enemies  or  capturing  prey,  of  specially  acute 
senses,   of  a  tendency  toward  concealment,  or  any  one  of  a 
thousand  things  calculated  to  fit  an  organism  for  a  special  place 
in  life.     It  is  not  necessary  to  suppose  that  these  elements  of  fit- 
ness exist  in  striking  degree  at  first.     The  struggle  is  so  intense 
that  even  the  slightest  handicap  may  mean  the  destruction  of 
the   individual.     This   elimination   of   the   weaker  individuals 
results  in  what  has  been  called  natural  selection  through  the 
"survival  of  the  fittest."     The  hereditary  qualities  thus  pre- 
served in  the  individual  are,  if  inherited,  subject  to  transmission 
by  heredity;  and  by  the  continuous  action  of  natural  selection 
and  heredity  through  a  long  series  of  generations  these  elements 
of  fitness  are  believed  to  accumulate,  and  thus  animals  become 
better  and  better  adapted  to  their  surroundings. 

138.  Artificial  Selection. — Since  man  has  been  on  the  earth 
he  has  been  a  most  potent  factor  in  the  environment  of  the  other 
animals.     He  has  helped  in  the  elimination  of  animals  hurtful  to 
his  interests;  has  domesticated  others  which  he  has  deemed 
useful,  thus  rendering  their  environment  highly  artificial  and 
removing  from  them  the  struggle  for  existence  in  certain  meas- 
ure.    For  natural  selection  he  has  substituted  a  conscious  selec- 
tion of  such  organisms  as  are  best  suited  to  his  needs  or  fancies, 


DIFFERENTIATION    OF   INDIVIDUALS   AND    ADAPTATION  105 

and  has  allowed  these  to  reproduce,  eliminating  the  others. 
This  artificial  process,  which  obtains  results  more  rapidly  than 
the  natural,  has  given  rise  to  the  various  breeds,  strains,  or 
races  of  dogs,  horses,  cattle,  fowls,  etc.  By  means  of  this  selec- 
tion the  habits  and  dispositions  of  the  domestic  animals  have 
been  improved  as  surely  as  their  structure.  Their  power  of 
self-support,  however,  has  been  so  materially  diminished  that 
some  of  them  could  not  succeed  in  finding  a  living  in  the  wild 
state  under  ordinary  circumstances. 

139.  Practical  Exercises. — Are  there  any  domesticated  animals  whose  species 
is  represented  in  the  wild  state?  Compare  the  habits  and  general  structure  of 
some  of  the  domesticated  animals  with  that  of  their  nearest  kin  among  wild  species. 
How  many  species  of  domestic  animals  can  you  enumerate?  From  what  groups 
do  they  come  ?  Trace  the  history  and  results  of  the  domestication  of  some  of  the 
common  animals,  as  fowls,  pigeons,  cats,  dogs,  etc.  Have  any  strictly  American 
species  been  domesticated  ? 

140.  The  Adaptation  of  Animals  to  their  Environment.— 

There  are  two  distinct  questions  of  importance  to  be  con- 
sidered in  connection  with  this  subject:  (i)  the  necessity  of  the 
adjustment  of  organisms  to  their  environment,  and  (2)  the 
means  by  which  this  adaptation  takes  place  in  the  individual  and 
becomes  fixed  in  the  species.  It  is  clear  that  the  limited  food 
supply  and  the  unlimited  powers  of  animals  to  reproduce  result 
in  a  struggle  for  food  among  the  animals  at  any  time  occupying 
the  earth  (135).  This  struggle  is  not  merely  among  the  animals 
in  question,  but  is  in  reality  between  every  organism  and  its 
whole  environment.  Extremes  of  heat  and  cold,  drouth  and 
famine,  and  numerous  changes  in  the  conditions  of  life  make  it 
absolutely  necessary  that  the  individual  shall  have  some  power  of 
adapting  itself  to  what  is  permanent  and  what  is  changeable  in  its 
environment.  What  are  the  means  then  by  which  animals  that 
are  not  completely  in  accord  with  their  surroundings  may  become 
so  ?  There  are  two  possible  ways  in  which  this  may  come  about. 
The  animals  may  migrate  to  regions  where  the  conditions  are 
naturally  more  favorable  to  their  well-being,  that  is,  to  regions  for 
which  they  are  already  adapted.  As  a  matter  of  fact  this  is 
known  to  be  a  common  occurrence.  Animals  often  disperse 
from  their  old  centre  of  multiplication  under  the  influence  of 


106  ZOOLOGY 

hunger  or  unfavorable  local  conditions.  They  are  often  assisted 
in  these  dispersals  by  such  natural  agencies  as  winds,  currents  of 
water,  and  by  other  animals.  If  the  migrating  forms  succeed  in 
finding  new  regions  suited  to  their  needs,  there  results  a  condi- 
tion of  adaptation  between  organisms  and  their  respective  envi- 
ronments, but  without  any  active  change  in  the  characteristics  of 
the  organism.  The  environment  itself  is  subject  to  continual 
change  and  there  are  too  many  barriers  in  the  way  of  universal 
migration  for  this  to  be  accepted  as  a  complete  explanation  of 
the  widely  observed  adjustment  of  animals  to  the  conditions 
which  surround  them. 

Again  animals  may  become  suited  to  their  environment  by 
variation,  without  migration.  There  is  no  question  that  this 
also  occurs  and  that  it  is  the  more  important  factor  of  the  two. 
It  has  been  shown  (133)  that  all  animals  are  variable.  Students 
of  biology  have  suggested  two  important  ways  in  which  varia- 
tions may  give  rise  to  a  harmony  between  the  organism  and  its 
surroundings.  This  result  may  take  place  through  natural 
selection,  which  eliminates  the  unfit  (137).  According  to  this 
view  the  organisms  naturally  tend  to  vary.  The  changing  en- 
vironment stimulates  this  tendency  to  variation.  Out  of  a 
thousand  individuals  of  similar  parentage  there  will  be  numer- 
ous slight  differences  of  structure  and  physiological  qualities. 
Some  of  these  will  be  more,  and  some  less,  favorable  to  the  en- 
vironment. In  the  struggle  those  will  be  eliminated  which 
for  any  reason  are  strikingly  unsuited  to  the  environment.  On 
the  other  hand,  those  animals  whose  variations  are  most  in 
accordance  with  the  local  condition  will  persist  and  propagate 
their  kind,  tending  through  heredity  to  pass  on  to  their  off- 
spring the  qualities  which  enabled  them  to  adjust  themselves 
to  their  surroundings.  Thus  there  will  be  a  gradual,  ever- 
increasing  adaptation  in  the  whole  species  of  which  they  are  a 
part,  by  natural  selection.  If,  however,  only  inherited  qualities 
are  transmitted,  and  the  fluctuations  of  body  produced  by  use 
and  environment  cannot  be  transmitted,  natural  selection  could 
not  act,  by  way  of  the  acquired  characters,  in  improving  a 
species. 

Occasionally  there  occurs  in  offspring,  from  the  action  of  the 


DIFFERENTIATION    OF   INDIVIDUALS    AND    ADAPTATION  1 07 

environment  or  from  other  causes,  a  sudden  and  considerable 
change  from  the  parent  type.  Such  a  product  is  known  as  a 
"sport."  It  is  quite  possible  that  natural  selection  may  seize 
on  such  and  if  in  a  favorable  direction  preserve  and  increase 
them.  In  such  cases  adaptation  might  take  place  with  great 
rapidity,  instead  of  in  the  gradual  way  described  above. 

In  the  second  place  it  has  been  argued  that  the  immediate 
effect  of  the  environment  on  the  organism  and  the  efforts  of  the 
organism  to  respond  to  the  stimuli  of  the  environment  produce  in 
the  organism  just  such  definite  variations  as  will  tend  to  fit  it  for 
its  surroundings.  In  other  words  the  majority  of  the  variations 
brought  about  by  a  given  external  condition  are  definite  and 
naturally  in  the  direction  to  meet  the  necessities  of  the  case.  For 
example,  cold  stimulates  the  surface  cells  of  the  body  of  an 
animal.  The  immediate  response  of  the  nervous  and  nutritive 
processes  in  the  organism  are  such  that  the  surface  cells  take  on 
greater  activity  and  produce  materials  at  the  surface  of  the  body 
which  tend  to  protect  the  animal  from  the  ill  effects  of  the  cold. 
This  is  an  individual  variation.  To  become  effective  in  making 
the  species  better  adapted  to  the  environment  these  results  must 
be  handed  down  by  inheritance  to  the  next  generation.  If 
this  can  take  place  this  theory  would  go  a  long  way  toward 
explaining  how  adaptations  arise.  There  is,  however,  very 
great  doubt  whether  such  adaptations  acquired  in  the  life  of 
an  individual  can  be  transmitted  to  offspring.  If  this  cannot 
occur  we  are  thrown  back  upon  natural  selection  as  the  prin- 
cipal explanation  thus  far  offered  to  account  for  the  progressive 
adaptation  of  animals  to  the  environment.  There  is  no  rea- 
sonable doubt  that  natural  selection  is  such  an  explanation. 
So  far  as  we  know  either  small  or  large  variations  may  be  se- 
lected if  they  are  only  inherited  variations  and  useful  ones. 
To  what  extent  it  is  assisted  by  other  factors  is  at  present  un- 
certain. It  will  be  assumed  in  the  following  pages  that  it  is  one 
of  the  most  important  known  factors  in  producing  adaptation. 

141.  Classification. — Since  the  environment  is  not  the  same 
at  any  two  places  on  the  earth  and  there  is  an  accumulation, 
from  generation  to  generation  in  animals,  of  those  features 


108  ZOOLOGY 

which  tend  to  bring  them  into  harmony  with  their  different 
environments,  it  is  inevitable  that  the  animals  themselves  come 
to  be  very  diverse,  no  matter  how  similar  they  were  at  the 
outset.  In  the  discussion  of  them  it  therefore  becomes  neces- 
sary to  devise  some  means  of  expressing  the  degree  of  likeness 
and  unlikeness  among  the  great  number  of  individual  animals 
existing  on  the  earth.  This  may  be  done  by  means  of  an  ap- 
propriate classification.  The  differences  of  structure  and  func- 
tion may  be  superficial  or  fundamental,  but  it  must  be  remem- 
bered that  all  these  differences  are  in  some  way  the  outcome 
of  the  history  of  the  organisms,  and  that  the  likenesses  are 
signs  of  kinship,  or  of  similar  history,  or  both.  The  grouping 
or  classifying  of  organisms  has  two  objects:  (i)  convenience, 
that  is,  to  make  future  work  easy ;  and  (2)  to  express  the  results  of 
past  study.  Insomuch  as  the  first  motive  may  predominate  the 
classification  may  be  artificial,  that  is,  may  bring  together  ani- 
mals that  are  really  not  closely  related,  though  possessing  a 
superficial  resemblance.  The  grouping  together  of  bats  and 
birds  on  the  ground  of  their  power  of  flying,  or  whales  with  fishes 
because  of  their  habitat,  would  illustrate  such  a  classification. 
In  proportion  as  classification  takes  in  all  the  facts  known  with 
regard  to  animals  and  expresses  the  relationship  of  forms  classed 
together,  it  is  said  to  be  natural.  Every  classification  is  in  some 
measure  artificial  since  we  do  not  know  all  the  facts  concerning 
the  structure  or  history  of  any  organism. 

142.  Terms  Used  in  Classification. — From  what  has  been 
said  concerning  the  power  of  multiplication  in  animals,  the  re- 
sulting struggle  for  existence,  the  variability,  and  the  elimination 
of  those  whose  variations  are  not  suited  to  the  various  environ- 
ments into  which  the  offspring  migrate,  it  will  be  readily  under- 
stood that  even  the  descendants  of  a  single  pair  of  organ- 
isms will  come  in  time  to  be  noticeably  different  in  form,  size, 
color,  and  the  like.  The  individuals  of  a  given  region  will  usu- 
ally be  more  like  each  other  than  like  their  cousins  who  have 
been  subjected  to  some  other  kind  of  environment.  There  is 
thus  a  need  of  terms  to  express  the  degree  of  difference  which, 
through  these  influences,  finally  characterizes  the  descendants 


DIFFERENTIATION   OF    INDIVIDUALS    AND    ADAPTATION  IOQ 

even  of  common  ancestors.  Such  groups  of  forms  are  usually 
known  as  varieties  or  subspecies  of  the  original  type  from  which 
they  all  sprang.  Thus  in  the  human  race  while  all  men  are  con- 
sidered as  belonging  to  one  common  type  and  possibly  derived 
from  the  same  human  ancestors  there  is  enough  difference  be- 
tween the  American  Indian  and  the  Caucasian  to  make  it  neces- 
sary to  distinguish  them  as  different  varieties.  Many  of  our 
widely  distributed  animals  as  the  dog,  the  horse,  the  common  fox 
have  varieties  which  are  readily  distinguishable. 

When  the  causes  which  produce  varieties  have  been  at  work 
long  enough  to  eliminate  the  intermediate  forms  which  are  often 
found  connecting  the  varieties,  and  to  secure  a  close  adaptation 
of  the  varieties  to  the  environment,  the  term  species  is  applied  to 
what  were  formerly  called  varieties.  Species  thus  merely  rep- 
resent the  further  progress  of  individual  differentiation  and  adap- 
tation to  the  different  modes  of  life  which  give  rise  to  variation  in 
individuals — that  is,  to  varieties.  A  species  of  animals  may 
again  split  up  by  the  action  of  the  forces  mentioned  (and  other 
conditions  which  have  not  been  mentioned)  into  new  varieties 
and  finally  into  new  species.  It  is  believed  that  the  present 
diversity  of  animal  and  plant  life  has  come  about  from  a  much 
more  limited  number  of  kinds  of  ancestors  by  a  method  essen- 
tially such  as  that  described  above.  The  student  will  realize 
that  in  nature  there  are  only  individuals.  There  is  really  no 
such  thing  in  nature  as  a  species.  This  is  purely,  a  mental  con- 
cept of  our  own. 

Varieties  of  the  same  species  usually  cross  freely.  Their 
offspring  are  usually  fertile.  The  individuals  of  different  spe- 
cies as  a  rule  cross  less  freely  and  when  they  do  cross  their  off- 
spring are  called  hybrids.  Hybrids  are  often  sexually  infertile. 

The  genus  is  related  to  species  somewhat  as  the  species  to 
the  varieties  which  compose  it.  A  genus  embraces  those  kin- 
dred species  which  show  a  high  degree  of  relationship  among 
themselves.  The  characters  which  serve  to  distinguish  differ- 
ent genera  are  more  fundamental  than  those  by  which  we 
recognize  varieties  or  species,  and  argue  a  more  extended  time 
in  the  differentiation  of  genera  than  is  required  for  species. 

Other  terms,  as  families,  orders,  classes,  phyla,  are  used  to 


110  ZOOLOGY 

denote  the  still  more  extensive  and  comprehensive  divisions 
of  the  animal  kingdom. 

143.  Illustration  of  Classification. — The  domestic  cat  has 
many  varieties  or  breeds,  as  the  maltese,  manx,  tortoise-shell, 
etc.     On  the  other  hand,  the  wild-cat,  the  tiger,  the  leopard, 
the  lion  have  numerous  points  of  structural  likeness  to  the 
domestic  cat,  and  are  said  to  be  species  belonging  to  the  same 
genus  (Felis).     The  genus  Felis  and   others  less  common  are 
placed  together  in  the  family  Felidcz.     These  with  the  members 
of  the  dog  family  and  others  constitute  the  order  Carnivora 
(flesh  eaters),  and  similarly  for  the  higher  groups  in  the  diagram 
below. 

Kingdom — Animalia  (Protozoa,  arthropods,  chordates). 
Phylum — Chordata  (fishes,  birds,  mammals). 

Class — Mammalia  (carnivora,  ruminants,  bats,  man). 
Order — Carnivora  (dogs,  wolves,  cats,  etc.). 
Family — Felidae  (cat  family). 

Genus — Felis  (cat,  lion,  tiger,  etc.). 

Species — Felis    domestica    (with    its    numerous 
varieties) . 

The  name  of  an  animal  is  its  generic  name  followed  by  its 
specific  name  as  above.  The  variety  name  is  added  when  there 
are  distinct  varieties. 

144.  Relation  of  the  Individual  to  the  Species. — The  vari- 
ous types  of  animals  produce  their  offspring  in  numbers  pro- 
portional to  the  difficulties  encountered  in  bringing  the  young  to 
maturity.     In    the    most  favorable  circumstances  many  more 
are  produced  than  can  survive.     In  cases  where  enemies  are 
numerous  millions  of  eggs  may  be  deposited  in  order  to  secure 
a  single  adult.     Nature  is  thus  said  to  be  lavish  in  her  waste 
of  individuals  in  order  that  the  species  may  be  continued  and 
improved  in  its   adaptations.     These   surviving   descendants, 
generation   after   generation,    have   become,    through   natural 
selection,  more  and  more  suited  to  their  surroundings.     This 
means  that  the  production  of  many  individuals,  a  large  num- 
ber of  which  never  reach  maturity,  secure  the  development  of 


DIFFERENTIATION   OF   INDIVIDUALS    AND    ADAPTATION  III 

a  small  aristocracy  which  propagates  the  type.  The  species 
is  related  to  its  individuals  something  as  the  individual  is  to 
the  renewed  and  changing  cells  of  which  it  is  composed.  Spe- 
cies are  not  constant,  but  even  the  most  fixed  must  undergo 
change  or  extinction  when  confronted  by  new  conditions. 
Species  however  are  less  variable  than  the  individuals  composing 
them  because  the  species  represents  an  average  condition 
of  all  its  individuals.  Adaptation  to  environment  is  the  great 
problem  which  every  animal  must  solve.  Those  which  do  solve 
it  successfully  constitute  the  species.  It  is  needful  then  to 
consider  next  those  characteristics  of  structure,  habit,  or  in- 
stinct whereby  a  species  of  organisms  becomes  successfully  ad- 
justed to  its  surroundings.  In  a  broad  sense  all  the  organs 
which  were  outlined  in  the  preceding  chapters  are  adaptations : 
the  digestive  organ  and  process,  to  the  nature  of  food;  the 
nervous  system  and  the  special  senses,  to  the  external  stimuli; 
the  lungs,  gills,  and  skin  to  the  need  of  oxygen,  and  the  like. 
In  contrast  to  adaptations  of  this  kind  we  now  consider  as 
adaptations  those  more  special  modifications  of  fundamental 
structure  by  which  a  species  becomes  more  suited  to  some  lim- 
ited habitat  or  to  some  special  mode  of  life  which  is  of  signal  use 
to  it  in  the  struggle  for  existence. 

145.    Classification  of  the  Principal  Types  of  Adaptation. 

A.  Adaptations  primarily  in  relation  to  the  inorganic  en- 
vironment. 

B.  Adaptations  primarily  related  to  other  organisms. 

I.  Among  animals  of  the  same  species. 

1.  Friendly  and  social, 
(a)  Mating. 

(6)   Parental  care  of  young. 

(c)  Organic  colonies. 

(d)  Social  and  communal  life. 

2.  Competitive:  for  food,   mates,   etc. 

II.  Among  animals  of  different  species. 
i.  Friendly  and  social. 

(a)  Commensalism. 
(6)  Symbiosis. 


112  ZOOLOGY 

2.  Competitive. 

(c)  The  predaceous  habit:  adaptations  for  offense 
and  defense. 

(d)  Parasitism. 

146.  Special  Adaptations  to  the  Inorganic  Environment— 

These  embrace  such  special  structural  devices  as  hair,  feathers, 
the  blubber  of  whales,  which  enable  the  body  to  maintain  its 
temperature  despite  the  condition  of  the  medium.  The  habits 
of  burrowing  and  hibernation,  the  winter  migrations  of  many 
animals^  especially  birds,  are  examples  of  instinctive  adaptation 
to  cold.  The  same  end  is  obtained  by  man  by  artificial  clothing, 
by  houses,  and  by  the  use  of  fire  which  has  been  one  of  the  most 
important  instruments  in  his  progress.  Rotifers,  infusoria,  and 
some  other  animals  have  become  capable  of  retaining  life  during 
thorough  drying,  and  of  resuming  activity  on  the  return  of  mois- 
ture. Adaptations  to  locomotion  in  different  media,  earth,  air, 
and  water;  to  climbing;  to  stationary  life,  belong  to  this  group. 
These  are  only  a  few  of  the  many  instances  of  adaptations  of 
organisms  to  the  'materials  and  the  forces  about  them.  It  is 
easy  to  see  that  some  of  the  adaptations  are  of  life  and  death 
importance,  and  without  them  the  species  would  become  extinct. 
It  is  believed  that  these  qualities  of  the  organism  arise  in  a  way 
something  like  this:  owing  to  the  irritability  of  all  protoplasm, 
the  prevalent  external  factors  as  heat,  light,  gravity,  moisture, 
and  chemically  active  substances  must  produce  some  change,— 
that  is,  some  response  on  the  part  of  the  organism.  Those  or- 
ganisms in  which  the  response  is  not  in  accordance  with  the  best 
adjustment  to  the  special  environment  are  less  likely  to  survive 
in  the  struggle  for  existence.  Those  which  do  survive  and  propa- 
gate their  kind  because  of  their  favorable  responses  to  these 
stimuli  are,  by  reason  of  these  facts,  more  and  more  likely  in 
succeeding  generations  to  possess  those  habits  and  structures 
suiting  them  to  their  surroundings. 

147.  Practical  Exercises. — Find  other  instances  which  seem  to  indicate  adapta- 
tion either  in  structure  or  habit  to  special  features  in  the  environment:  as  adapta- 
tions to  prevent  undue  evaporation  in  a  dry  climate;  adaptations  to  warm  condi- 
tions; to  drouth;  to  the  use  of  special  plants  as  food;  to  light;  to  gravity.  Illustrate 
from  observation  and  by  library  reference  the  types  of  adaptations  cited  in  the  text 


DIFFERENTIATION   OF    INDIVIDUALS    AND    ADAPTATION  113 

above.  Is  the  power  of  sleeping  an  adaptation  of  any  value?  Among  what 
animals  is  it  found  ?  Find  instances  in  which  useful  adaptations  have  become 
useless  and  even  hurtful  from  changed  conditions  of  life. 

FIG.  52. 


FIG.  52.     Young  Opossum  (Dideiphys  virginiana)  photographed  from  life  by  J.  W.  Folsom. 

Questions  on  the  figure. — Of  what  conceivable  value  to  the  animal  is  the 
prehensile  tail?  In  what  other  groups  of  animals  is  the  tail  prehensile?  What  are 
the  habits  of  the  opossum  ?  How  is  this  species  distributed  on  the  earth  ?  Where 
are  other  marsupials  found  ? 

148.  The  relations  of  animals  of  the  same  species  to  one 
another  is  an  interesting  mixture  of  competition  and  coopera- 
tion. In  the  higher  forms  the  parents  instinctively  make  great 
personal  sacrifices  that  the  offspring  may  be  cared  for;  the  off- 
spring on  the  other  hand  struggle  with  each  other  for  this 
parental  provision.  In  the  classification  offered  (145)  it  should 
be  remembered  that  both  friendly  and  competitive  habits  and 
structures  are  always  represented  in  the  same  individual. 


114  ZOOLOGY 

149.  Mating  Adaptations. — One  of  the  most  striking  forms 
of  individual  variation  is  seen  in  the  differences  between  the 
sexes  of  higher  animals.  The  male  and  female  are  often  so 
widely  different  in  form,  size,  color,  and  other  qualities,  that 
naturalists  have  classified  them  as  belonging  to  different  species 
and  yet  it  is  very  manifest  that,  though  different,  the  sexes  are 
closely  adapted  to  each  other.  In  the  lower  types  of  animals 
the  sexes  are  frequently  represented  in  the  same  individual.  In 
such  cases  the  elements  often  mature  at  different  times.  An 
individual  is  thus  alternately  male  and  female.  This  is  re- 
garded by  many  as  being  the  primitive  condition, — the  separa- 
tion of  the  sexes  being  accomplished  by  the  repression,  so  to 
speak,  of  one  or  the  other  sex  in  each  individual.  Some  think 
that  the  temperature  and  the  amount  and  quality  of  food  have 
something  to  do  with  the  proportion  of  males  and  females  which 
are  produced.  So  sexual  dimorphism  in  some  species  may  be 
in  some  measure  a  response  to  external  conditions  and  presents 
every  evidence  of  being  an  advantageous  adaptation  to  the  con- 
ditions of  life.  On  the  other  hand,  it  is  believed  by  many  that 
the  sex  of  most  organisms  is  determined  by  conditions  in  the 
egg  and  sperm  that  unite.  In  other  words  it  is  thought  that 
sex  is  inherited,  and  cannot  be  changed  by  external  conditions. 
The  very  union  of  the  sperm  and  the  ovum,  whereby  two  cells 
lose  their  individuality  in  one,  with  a  renewal  of  powers  and  the 
mingling  of  the  qualities  of  two  parents,  must  be  looked  on  as  an 
adaptation  of  the  very  highest  moment  to  the  animals  in  which 
it  first  appeared  and  to  their  descendants.  So  too  are  the  won- 
derful internal  tendencies  that  cause  the  definite  unions  and 
separations  of  chromosomes  in  the  sex  cells.  The  chemical 
attraction  which  the  female  cell  exerts  on  the  motile  sperm  cell 
is  a  special  adaptation  to  accomplish  this  union.  Furthermore 
it  is  undoubtedly  true  that  many  of  the  color  markings,  notes, 
motions,  and  the  like  in  which  the  male  and  female  animals 
differ  are  recognition  marks  whereby  the  presence  of  one  sex  is 
made  known  to  the  other.  In  some  animals  in  which  sexual 
fertilization  normally  occurs,  the  ova  may  develop  in  the  absence 
of  sperm  (parthenogenesis) .  This  may  have  arisen  as  an  adapta- 
tion to  temporary  scarcity  of  males.  This  view  is  in  some  cases 


DIFFERENTIATION    OF   INDIVIDUALS   AND    ADAPTATION  115 

supported  by   the  additional  fact  that  parthenogenetic  eggs 
produce  male  individuals  wholly,  or  in  excess. 

150.  Practical  Exercises. — What  is  the  difference  in  the  notes  of  the  male  and 
female  of  the  American  quail  which  would  serve  as  recognition  marks?  Mention 
other  cases  of  sexual  dimorphism  which  appear  to  you  to  serve  a  similar  end. 
What  evidences  have  we  that  the  mingling  of  sperm  and  ovum  results  in  a  re- 
juvenescence? in  the  introduction  of  greater  variation?  Show  how  these  are 
important  as  adaptations  in  the  struggle  for  existence.  In  what  groups  is  partheno- 
genesis found  ?  Give  details  of  the  facts  in  several  cases. 

151.  Reproduction  and  Care  of  Young. — The  very  rate  of 
reproduction  is  an  adaptation  to  the  severity  of  the  struggle  for 
existence  experienced  by  the  animals  of  a  given  species.  Those 
forms  with  few  enemies  and  abundant  food  usually  need  to  pro- 
duce only  a  few  young  in  order  to  maintain  their  place.  Others 
less  favored  in  these  regards,  as  many  insects,  the  lobster,  the 
salmon,  must  reproduce  thousands  of  young  in  a  lifetime.  Simi- 
larly the  length  of  the  reproductive  period  and  of  life  becomes 
an  adaptation  to  the  same  end.  It  is  clear  from  these  facts 
that  any  device  which  the  parent  may  adopt  likely  to  bring  a 
larger  percentage  of  the  young  to  maturity  will  make  for  a 
saving  in  the  necessary  birthrate.  This  husbands  the  parental 
resources  and  conduces  to  the  efficiency  of  the  individual  and 
of  the  species.  It  must  not  be  supposed  that  parental  care  is 
confined  to  the  higher  animals.  In  its  most  elementary  condi- 
tion it  takes  the  form  of  food  stored  in  the  egg,  and  in  depositing 
the  egg  in  a  safe  place  for  hatching.  After  hatching  it  takes  the 
form  of  supplying  food,  or  protection,  or  both.  Cephalopods, 
fishes,  and  birds  have  a  large  amount  of  food  substance  stored  in 
the  egg.  Many  animals,  as  the  clam,  some  fishes,  some  reptiles, 
and  the  mammals,  retain  the  eggs  in  special  portions  of  the 
body  until  development  has  well  begun.  The  flies  lay  their 
eggs  in  the  decaying  matter  which  the  young  use  as  food.  The 
solitary  wasps  seal  theirs  up  in  nests  with  the  food  (dead  or 
wounded  spiders  or  insects)  on  which  they  are  to  develop. 
Other  insects  bore  into  the  tissues  of  living  plants  and  deposit 
their  eggs,  about  which  "galls"  or  masses  of  abnormal  vegetable 
tissue  are  developed.  The  ichneumon  fly  deposits  its  eggs  in 
the  body  of  some  other  animal.  Thus  we  see  an  immense  num- 


n6 


ZOOLOGY 


FlG.  53- 


FlG.  53.     Galls  on  oak,  cynipid  (Holcaspis  duricoria).     Natural  size.     Photo  by  Folsom. 


FIG.  54. 


FIG.   54.      Galls  on  elm,  produced  by  an  aphid,  Colopha  ulmicola.     Natural  size.     Photo  by 

J.  W.  Folsom. 


DIFFERENTIATION    OF    INDIVIDUALS   AND    ADAPTATION  117 

her  of  adaptations  useful  to  the  organism  have  been  developed 
in  connection  with  the  egg-laying  habit.  After  such  provision 
the  majority  of  animals  leave  the  young  to  care  for  themselves; 
but  many  higher  forms  take  further  pains  to  protect  and  train 
their  offspring  during  the  course  of  their  development.  The 
care  which  the  birds  and  mammals  give  their  young  is  a  matter 
of  common  observation.  It  takes  the  form  of  food,  of  special 
homes, — as  nests,  burrows,  dens,  etc.,  and  of  the  personal  ser- 
vices of  the  parents,  who  will  often  protect  the  young  from  their 

FIG.  55- 


FIG.  55-     Galls  on  hackberry  leaf,  produced  by  a  fly  (Cecidomyiida).     Natural  size. 
Photo  by  Folsom. 

Questions  on  figures  53,  54,  55. — What  does  the  gall  represent  from  the  point 
of  view  of  the  plant?  From  the  point  of  view  of  the  insect?  What  seems  to 
cause  the  undue  vegetable  growth?  Find  other  galls  in  nature  and  try  to  find 
what  type  of  insect  is  responsible  for  them?  In  what  ways  may  one  hope  to 
determine  this  fact? 

enemies  at  the  risk  of  their  own  life.  Similar  care  is  shown  by 
some  insects,  especially  the  social  forms,  such  as  bees,  ants,  and 
the  like.  The  lobster  carries  its  young  on  its  abdominal  ap- 
pendages for  months  after  hatching.  The  lower  invertebrates 
are  practically  destitute  of  these  later  care-taking  instincts. 

It  is  interesting  to  notice  that  animals  differ  very  much  in 
their  helplessness  at  hatching  or  at  birth.  The  young  of  the 
reptiles,  or  the  duck,  or  the  chicken  are  relatively  well  developed 
at  hatching,  and  are  very  soon  able  to  run  about  and  feed  (pre- 


u8 


ZOOLOGY 


cocial).  The  young  of  the  song  birds,  as  the  thrushes,  swallows, 
etc.,  are  wholly  dependent  on  the  care  of  the  parents  for  a  con- 
siderable time  (altricial).  In  the  herbivorous  mammals,  as  the 
sheep  and  cattle,  the  young  have  the  use  of  their  limbs  in  a  short 
time  after  birth.  Among  the  carnivorous  forms,  as  the  cat  and 
dog,  the  young  are  more  helpless.  In  the  human  species  the 
period  of  helplessness  is  longest  and  consequently  the  necessity 

FIG.  56. 


FIG.  56.     Nestling  Marsh  Hawks  (Circus  cyaneus).     From  Year-Book.     Department  of 

Agriculture. 

Questions  on  the  figure. — What  are  the  nesting  and  breeding  habits  of  the 
marsh  hawk?     Are  the  young  precocial  or  altricial? 

of  parental  care  greatest.  In  general,  the  longer  period  of 
parental  protection  accompanies  the  development  of  more  com- 
plex and  highly  organized  instincts,  and  intelligence.  The 
lengthened  period  of  dependence,  while  a  burden  to  the  parent 
in  one  sense,  is  an  advantage  to  it  in  the  saving  in  number  of 
offspring,  and  serves  to  benefit  the  species,  not  merely  by  keep- 
ing the  offspring  alive  until  they  may  reproduce,  but  in  the 
greater  development  of  such  parental  instincts  as  gentleness, 
self-sacrifice,  and  the  like.  In  the  human  race  it  has  given  rise 


DIFFERENTIATION    OF   INDIVIDUALS    AND    ADAPTATION  IIQ 

to  the  home  and  family,  which  we  regard  as  the  real  basis  of 
modern  society ;  and  such  social  organization  in  turn  has  been  a 
most  powerful  factor  in  the  progress  of  the  human  species. 
Death  and  the  length  of  life  must  also  be  considered  as  special 
adaptations.  This  differs  in  different  species  very  widely. 
Life  in  general,  where  natural  selection  acts,  will  be  the  period 
of  youth,  plus  the  period  of  fertility,  plus  the  time  necessary 
to  rear  the  latest  offspring.  For  the  species,  the  death  of  the 
individual  becomes  an  advantage  at  the  completion  of  this 
period,  and  this  fact  is  sufficient  to  insure  that  death  will  nor- 
mally occur  at  this  time. 

152.  Practical  Exercises. — Add  instances  of  parental  care  which  have  fallen 
under  your  own  observation,  and  give  a  statement  of  the  facts  in  the  case.  Com- 
pare the  mammals  with  which  you  are  acquainted,  in  this  regard.  Compare  the 
condition  of  the  young  of  the  robin,  the  quail,  the  blue-jay,  the  pigeon  as  to 
maturity  at  hatching.  Do  any  animals  of  your  acquaintance  reproduce  more  than 
once  in  a  year?  Why  is  one  reproductive  period  per  year  a  common  adaptation. 
Compile  statistics  concerning  the  longevity  of  various  animals,  and  its  relation  to 
size,  to  reproductive  period,  and  to  the  time  demanded  to  reach  the  adult  stage. 

153.  Colonies. — In  some  of  the  lower  groups  of  animals, 
as  the  polyps  and  jelly-fishes,  in  which  the  reproduction  by 
fission  or  budding  is  prominent,  the  newly  formed  individuals 
remain  for  a  longer  or  shorter  time  in  association  with  the  parent 
or  with  each  other.  These  units  which  otherwise  might  be 
separate  individuals  are  originally  connected  and  often  come, 
by  the  continuation  of  the  process,  to  form  immense  masses, 
as  in  the  coral.  Such  organic  associations  are  called  colonies, 
Colonies  rarely  occur  in  animals  in  which  the  organs  are  highly 
specialized.  Very  often  the  individuals  become  specialized  for 
the  performance  of  a  special  portion  of  the  work,  and  thus  we 
get  several  quite  differently  constructed  individuals  within  the 
colony  (polymorphism,  Fig.  87).  The  whole  colony  may  then 
behave  somewhat  as  an  individual,  the  polyps  taking  the  place 
of  organs  (Siphonophora) .  Colonial  animals  are  almost  always 
attached  to  fixed  or  floating  objects.  These  polymorphic  indi- 
viduals are  closely  adapted  to  each  other  in  structure  and  divi- 
sion of  labor;  and  the  colonial  habit  in  general,  even  where  there 
is  no  division  of  labor,  is  a  successful  device  whereby  limited 
areas  are  completely  occupied  by  the  members  of  a  species  (as 


120  ZOOLOGY 

in  the  case  of  the  branching  corals)  where  the  single  polyps  would 
be  practically  helpless.  The  arrangement  of  the  polyps  on  the 
common  skeleton  and  the  rate  of  growth  of  the  different  polyps 
are  beautifully  adapted  to  the  best  use  of  the  currents  of  water 
by  which  the  food  and  oxygen  are  conveyed. 

154.  Library  and  Museum  Exercise. — What  phyla  of  the  animal  kingdom 
supply  instances  of  colonies?  Trace  different  degrees  of  polymorphism.  In 
what  different  ways  do  the  individuals  occur  on  the  common  stock?  Show  how 
the  relative  rate  of  growth  of  the  differently  placed  individuals  determines  the 
ultimate  form  of  the  colony  as  a  whole. 

155.  Social  and  Communal  Life. — Animals  of  the  same 
species  often  become  associated  even  when  there  is  no  organic 
connection  between  the  individuals.  The  association  may  be 
temporary  or  permanent.  The  bond  in  these  cases  is  not  phys- 
ical, but  instinctive  and  psychical.  In  its  simplest  form  this  is 
merely  a  matter  of  gregariousness  such  as  is  seen  in  the  schools  of 
fishes  or  flocks  of  birds,  which  are  apparently  brought  together 
at  certain  periods  by  a  common  instinct  or  by  common  needs. 
A  step  more  intimate  is  the  banding  together  of  predaceous 
animals  as  wolves  or  vultures,  or  pelicans,  for  mutual  help  in 
finding  or  capturing  the  prey.  Corresponding  to  this,  on  the 
part  of  their  victims,  we  find  the  herding  of  the  bison,  of  deer, 
and  their  allies  for  protection,  whether  by  fighting  together  or 
by  the  stationing  of  sentinels  to  give  notice  to  the  feeding  herd 
of  the  approach  of  danger.  In  still  other  forms,  notably  among 
such  insects  as  the  bees  and  ants,  there  is  a  very  intimate  and 
permanent  union  in  social  life.  This  is  usually  associated  with 
the  instinct  of  home  building,  and  thus  a  high  degree  of  division 
of  labor  with  its  great  advantages  becomes  possible.  This  is 
carried  to  such  an  extent  that  often  polymorphic  individuals 
result,  much  as  in  the  organic  colonies.  In  such  cases  it  is  clear 
that  the  individual  life  comes  to  be  bound  up  in  the  success  of 
the  community.  Such  forms  usually  exert  great  care  for  their 
young  and  develop  a  relatively  high  order  of  "intelligence." 
The  principal  social  forms  are  the  ants,  of  which  there  are  more 
than  two  thousand  species;  some  of  the  bees  and  wasps;  the 
termites,  or  so-called  white-ants;  beavers;  some  monkeys  and 
man. 


DIFFERENTIATION    OF   INDIVIDUALS    AND    ADAPTATION  121 

156.  Library  Exercise. — Make  a  report  on  the  social  life  of  the  honey-bee, 
including  the  following  points:  the  home;  the  kinds  of  individuals,  their  origin, 
and  their  work  in  the  community;  their  food  and  its  preparation;  mode  of  caring 
for  the  young;  swarming  and  its  significance.  Make  a  similar  report  concerning 
some  species  of  ant.  Find  facts  concerning  the  following  topics:  "ants'  cows"; 
slave-making  among  the  ants;  army  ants;  the  agricultural  ant. 

157.  Competition  among  animals  of  the  same  species  is  not, 
for  the  most  part,  of  a  personal  character  except  in  the  case  of 
the  struggles  of  the  males  of  polygamous  animals.  The  ordinary 
struggle  for  existence  among  them  is  merely  that  of  food-seeking, 
where  all  possess  the  same  organs  and  habits  but  in  varying 
degrees  of  excellence.  Those  which  have  the  greater  strength, 
hardiness,  or  intelligence  are  more  likely  to  get  their  portion  of 
food  at  the  expense  of  the  weaker,  and  thus  to  propagate  their 
qualities.  Sometimes,  however,  animals  live  directly  at  the  ex- 
pense of  their  own  species.  Young  spiders  before  escaping  from 
the  cocoon  in  which  they  are  hatched  devour  each  other,  thus 
instituting  an  acute  phase  of  the  struggle  for  existence  in  the 
place  of  the  protection  prepared  by  parental  care.  Many  fishes 
are  known  to  devour  their  own  young.  We  have  all  had  occa- 
sion to  wonder  what  becomes  of  the  small  frogs  in  a  box  contain- 
ing large  ones. 

The  struggle  between  the  males  for  the  possession  of  the 
females  has  resulted  in  the  development  of  many  interesting 
adaptations.  The  struggle  may  take  the  form  of  actual  combat 
in  connection  with  which  organs  of  offense  and  defense  are 
found.  Such  are  the  horns,  tusks,  spurs,  manes,  and  even  the 
excessive  size  of  the  males  as  compared  with  the  females.  Mani- 
festly the  same  qualities  which  make  a  male  a  formidable  rival 
to  another  are  likely  to  be  of  service  to  himself,  his  mates,  and 
his  young,  and  thus  to  the  species,  in  protecting  them  from  the 
attack  of  their  enemies  among  other  species.  The  competition 
between  males  is  not  all  of  this  stressful  kind,  however.  It  is 
believed  by  many  naturalists  that,  in  those  instances  where 
simple  mating  rules,  those  males  with  the  most  striking  colors, 
pleasant  voices,  and  winning  ways  displace  their  less  favored 
rivals  and  thus  tend  to  accumulate  by  natural  (sexual)  selection 
the  adaptations  of  this  class. 


122  ZOOLOGY 

158.  The  individuals  of  one  species  of  animals   may  often 
be  practically  indifferent  to  the  presence  of  those  of  other  species. 
Their  relation  is  simply  that  of  competing  for  the  general  food 
supply  and  thus  assisting  in  the  elimination  of  the  unfit  in  all 
species.     They  may  graze  in  the  same  pasture,  swim  in  the  same 
pool,  or  even  be  parasitic  on  the  same  host,  and  have  no  other 
relation.     From  this  as  the  simplest  relationship  we  may  pass 
by  gradual  stages  to  the  most  intimate  friendships  and  the  most 
bitter  antagonism.     Every  species  is  indifferent  to  some  and 
hostile  to  other  of  the  species  which  surround  it;  and  man  is  no 
exception  to  the  rule.     It  is  a  perversion  of  manifest  fact  to 
pretend  that  all  animals  are  of  some  use  to  man. 

159.  We  have  seen  that  the  individuals  of  a  given  species 
are  engaged  in  a  struggle  among  themselves  for  the  means  of 
subsistence,  and  that  in  certain  cases  they  form  communities 
or  colonies — a  kind  of  organic  corporation — in  order  to  meet 
more  successfully  the  demands  made  upon  them  by  their  en- 
vironment.    Similar  partnerships  may  be  formed  by  animals 
of  different   species.     The  simplest  of  these  associations  are 
known  as  commensalism  or  "  mess-mateism,"  in  which  the  degree 
of  dependence  and  mutual  advantage  is  perhaps  not  very  great. 
As  instances  may  be  cited  the  occupancy  of  the  same  burrows 
by  the  prairie  dog  and  a  species  of  owl;  the  attachment  of 
barnacles  to  whales  and  sharks;  the  hundreds  of  species  of 
other  insects  which  live  in  the  nests  of  ants;  the  lodging  of 
fishes  and  other  animals  in  the  body-cavity  of  some  of  the 
large  tropical  sea-anemones  or  among  the  tentacles  of  some 
of  the  Hydrozoa.     Each  member  of  the  association  can  live 
without  the  other,  but  for  some  reason  they  often  occur  to- 
gether.    The  way  in  which  species  of  rats  and  mice  follow 
man  and  occupy  his  habitations  perhaps  may  be  considered 
as  illustrating  a  similar  condition. 

1 60.  Symbiosis. — Under  this  term  are  included  even  closer 
relationships  between  members  of  different  species,  where  there 
seems  to  be  a  distinct  advantage  accruing  to  both  members  of 
the   partnership    sufficient    to    account    for   it.     The   relation 
of  the  ants  to  the  aphides  or  plant-lice  which  they  capture  may 


DIFFERENTIATION    OF   INDIVIDUALS    AND    ADAPTATION 


123 


be  so  described.  The  aphides,  although  captives,  are  nour- 
ished, often  at  great  expense  of  labor  to  the  ant,  on  the  food 
which  they  most  prefer,  and  in  return  the  ants  use  the  sweet 
secretions  of  their  bodies  as  food.  Certain  hermit-crabs, 
whose  habit  it  is  to  occupy  gasteropod  shells  as  a  home  into 
which  they  insert  the  soft  posterior  part  of  the  body,  cultivate 
friendly  relations  with  a  sea-anemone  which  becomes  attached 
to  the  shell,  often  with  the  active  help  of  the  crab.  The 
anemone  is  supposed  by  some  to  conceal  the  hermit  and  to  help 

FIG.  57. 


FIG.  57.     Hermit-crab  in  the  shell  of  a  Gasteropod.     After  Morse. 

Questions  on  the  figure.^-What  structural  adaptations  has  the  hermit-crab 
to  this  mode  of  life?  What  conceivable  gain  has  such  a  habit?  What  animals 
are  cited  as  symbiotic  with  the  hermit-crab  ? 

protect  it  by  means  of  its  nettling  cells,  and  in  return  is  carried 
about  to  fresh  fields,  and  enjoys  a  portion  of  the  food  broken 
up  by  the  strong  pincers  of  the  crab.  Observers  have  claimed 
that  the  crab  offers  choice  morsels  of  food  to  its  companion. 
When  the  crab  by  reason  of  its  growth  needs  a  new  home  it  is 
said  to  transplant  the  anemone  thereto.  These  must  be  looked 
upon  as  very  remarkable  adaptive  instincts.  Symbiosis  is 
probably  more  common  between  animals  and  plants  than 
among  animals.  The  most  interesting  of  these  latter  are  seen 
in  the  so-called  "ant-loving"  plants,  in  which  the  plant  pro- 


124 


ZOOLOGY 


duces  special  homes  or  special  foods  for  the  ants,  and  the  ants 
in  return  protect  the  plant  from  the  ravages  of  other  leaf- 
cutting  ants  or  hurtful  insects.  Certain  sea- anemones  possess 
unicellular  algae  imbedded  in  the  cells  of  the  entoderm.  These 
algae  derive  their  nourishment  from  the  wastes  of  the  animal 
tissues  and  supply  oxygen  and  possibly  other  matter  to  the 
cells  in  which  they  lie.  The  close  relation  between  the  struc- 

FIG.  58. 


FiG.  58.     Argynnis  cybele  on  thistle.     Natural  size.     Photo  by  Folsom. 

Questions  on  the  figure. — For  what  purpose  does  the  butter  fly  visit  the 
thistle?  What  special  adaptations  does  the  butter  fly  possess  for  this  mode  of  life? 
What  is  the  gain  to  the  thistle  from  the  visits  ? 

ture  and  instincts  of  insects,  on  the  one  hand,  and  the  form  of 
flowers,  their  products  and  needs,  on  the  other,  illustrates  a 
symbiotic  adaptation  which  has  long  attracted  students  both 
of  botany  and  zoology.  See  Fig.  58. 

161.  Library  Studies. — Make  a  report  concerning  the  various  myrmecophilous 
plants.  Accumulate  all  the  supposed  instances  of  symbiosis  which  your  library 
records.  Lichens,  among  plants,  are  considered  to  illustrate  symbiosis.  How? 


DIFFERENTIATION    OF   INDIVIDUALS   AND   ADAPTATION  125 

162.  The   Preying   Habit. — The   effects   of   this   habit   are 
stamped  upon  the  structure  and  activities  of  both  the  pursuer 
and  the  pursued.     It  is  in  this  relation  that  nature  is  indeed 
"red  in  tooth  and  claw."     While  in  general  the  same  organs 
and  habits  which  are  of  value  in  the  capture  of  prey  are  useful 
in  the  defense  of  the  possessor,  it  is  possible  to  find  a  series 
of    adaptations   of   an    offensive   character   and   others    more 
specially   of   defensive   value.     The   curved   claws   and   sharp 
teeth,  the  stealthy  approach,  the  sudden  spring,  and  the  great 
agility  of  the  one  are  met  by  the  timidity,  the  keen  senses, 
the  fleetness  of  the  other.     We  can  see  that  these  defensive 
adaptations  must  keep  pace  with  the  offensive  else  the  prey 
would  be  exterminated,  which  would  entail  no  less  surely  the 
destruction   of   their  enemies   than  if   these  should  lose  their 
power  of  capturing  their  prey. 

163.  Adaptations  for  Protection. — In  addition  to  the  alter- 
natives of  fighting  or  fleeing,  the  animals  which  are  preyed 
upon  have  very  interesting  and  effective  qualities  that  make 
for  safety.     Many  forms,   as  the  Crustacea,   have  permanent 
outer  coverings;  most  mollusks  have  a  box  arrangement  into 
which  they  can  retire  when  threatened  by  attack;  others  by 
burrowing  or  otherwise  come  to  occupy  obscure  corners  in 
nature  where  enemies  find  it   difficult  to  follow.     Forms  as 
widely  different  as  the  earth-worm,  mole,  and  the  chamois  find 
safety  in  retirement. 

This  hiding-theme  may  be  wrought  out  in  ways  almost 
equally  effective  by  what  is  called  protective  resemblance.  By 
this  is  meant  that  the  animal  becomes  less  easily  distinguished 
from  its  environment  because  of  its  color,  or  form,  or  both. 
This  resemblance  may  be  to  some  particular  object,  or  merely 
a  general  harmony  of  color  with  the  surroundings.  As  illus- 
trative of  the  latter  head  we  may  cite  the  quail  among  the 
dead  leaves  and  grasses,  the  sober-hued  lizard  on  the  logs,  the 
green  caterpillars  or  tree-toads  among  the  leaves;  the  tawny 
color  of  desert  animals,  the  white  fur  of  arctic  forms,  the  trans- 
parency of  many  marine  animals.  Indeed  the  great  majority 
of  animals  show  some  traces  of  resemblance  to  the  surround- 


126 


FIG.  59. 


Nestling  Mourning  Doves  (Zenaidura  macroura). 
Year-Book,  1900. 


From  U.  S.  Dept.  Agriculture 


Questions  on  the  figure. — Is  there  anything  suggestive  of  protective  markings? 
What  are  the  nesting  habits  of  the  dove?     What  character  of  nest  is  constructed? 


FIG.  60. 


FIG.  60.     A  sea-horse, — Phyllopteryx  eques.     From  Eckstein. 

Questions  on  the  figure. — Compare  this  figure  of  sea-horse  with  figures  of 
other  species  and  note  the  chief  difference  between  them  and  the  typical  fishes  in 
external  characteristics.  What  about  the  figure  suggests  protective  resemblance? 
At  what  point  does  the  tail  of  the  fish  end  ? 


DIFFERENTIATION   OF   INDIVIDUALS    AND    ADAPTATION 


12 


ings,  since  concealment  is  alike  advantageous  to  the  predaceous 
and  to  their  prey.  In  some  instances  there  is  the  ability  to 
change  color  with  changing  environment,  as  in  the  tree-toads, 
the  chameleon,  and  in  some  fishes.  This  is  not  chiefly  by  the 
direct  action  of  the  light  on  the  pigment  cells  but  by  reflex 
action  of  the  nervous  system  stimulated  through  the  eyes. 

Many  other  animals  become  inconspicuous  by  reason  of  a 
resemblance  to  special  objects.  It  is  among  the  insects  that  the 
most  numerous  illustrations  of  this  are  found.  The  walking- 
stick  insect  appears  as  dead  twigs  when  not  in  motion.  Many 
butterflies  resemble  leaves  when  at  rest.  A  noted  instance  is 
Kallima  which  is  a  large  species,  conspicuous  when  flying  be- 


FIG.  61 


FIG.  61.     Walking-stick  insect  (Diapheromera  veliei)  on  twig.     Natural   size.     By  J.  W.  Folsom. 

Questions  on  the  figure. — To  what  group  of  insects  does  this  belong  ?     Do  you 
see  any  reason  to  suppose  that  it  illustrates  protective  resemblance? 

cause  of  blue  and  orange  patches  on  the  upper  surface  of  the 
wings.  The  wings  are  folded  when  at  rest  and  the  lower  sides 
are  colored  and  marked  so  like  a  dead  leaf  that  the  deception, 
is  very  complete.  The  larvae  of  some  of  the  geometrid  moths, 
often  called  "measuring- worms,"  are  remarkably  like  the 
twigs  on  which  they  crawl,  both  in  color  and  shape.  This  is 
made  more  striking  by  the  presence  of  roughnesses  on  the  sur- 
face which  suggest  buds,  and  by  the  possession  of  muscles 


128  ZOOLOGY 

which  enable  them  to  support  themselves  rigidly  outstretched 
for  hours  by  means  of  the  posterior  legs  alone,  so  that  the 
axis  of  the  body  makes  an  angle  with  the  branch. 

Other  instances  of  special  devices  whereby  animals  protect 
themselves  are  found  in  the  electric  organs  of  some  eels  and 
other  fishes,  in  the  poisonous  fluids  with  or  without  special 
stinging  organs,  as  in  ccelenterates,  bees,  some  spiders,  a  few 
fishes  (spines) ,  and  some  snakes ;  also  in  the  repulsive  odors  of  the 
skunk  and  many  caterpillars. 

Caterpillars  oftentimes  have  an  acrid  or  otherwise  unpleasant 
taste,  but,  unless  this  is  associated  with  a  special  odor  or  color 
by  which  its  enemies  may  recognize  the  fact,  it  is  not  likely  to 
prove  of  any  great  service  to  the  animal  possessing  it  since  a  sin- 
gle incision  in  the  soft  body  made  by  the  bill  of  a  bird  is  likely  to 
.cause  death.  For  similar  reasons  animals  with  stings  are  often 
highly  colored.  The  colors  or  other  marks  are,  in  these  cases,  in 
the  nature  of  warnings.  The  "monarch,"  one  of  our  large  con- 
spicuous butterflies  is  an  illustration  of  the  association  of  color 
and  offensive  taste;  the  wasps  and  the  coral-snake,  of  the  asso- 
ciation of  calor  with  the  possession  of  stinging  powers.  Thus 
owing  .to  the  power  of  association  in  the  mind  of  the  enemies, 
the  advantage  comes  to  lie  quite  as  much  in  the  possession  of 
the  special  color  or  form  as  in  the  presence  of  the  underlying 
protective  powers.  These  facts  give  rise  to  the  remarkable 
phenomena  of  mimicry.  This  term  applies  to  those  instances 
where  an  edible  or  harmless  animal,  by  reason  of  its  similarity 
to  those  which  are  disagreeable,  partakes  of  their  immunity 
from  attack.  Mimicry  must  not  be  considered  as  in  any  way 
a  matter  of  choice  with  the  animal  but  simply  the  result  of 
natural  selection  in  preserving  and  allowing  the  propagation 
of  favorable  variations.  The  viceroy  butterfly,  though  edible, 
seems  to  be  protected  by  its  striking  likeness  to  the  monarch. 
The  nearest  relatives  of  the  viceroy  are  quite  differently  marked. 
Mimicry  of  bees  and  wasps  is  found  among  many  flies  and  some 
moths  and  beetles.  Non-venomous  snakes  occasionally  have 
the  marking  and  the  motions  of  the  venomous. 

164.  Practical  Exercise. — Try  to  discover  instances  of  general  protective  re- 
semblance among  the  animals  known  to  you.     Analyze  each  case  and  see  just 


DIFFERENTIATION   OF   INDIVIDUALS   AND   ADAPTATION  129 

the  nature  and  value  of  the  protection.  Treat  similarly  the  subject  of  special  pro- 
tective resemblance.  Do  you  know  any  really  harmless  animals  which  assume 
apparently  dangerous  attitudes  for  protection?  Accumulate  all  the  available 
references  on  mimicry.  What  range  of  color  have  you  seen  illustrated  among 
animals?  In  a  single  animal?  Where,  on  the  earth,  are  the  brightest-hued 
animals  found?  What  are  believed  to  be  the  causes  of  colors  among  animals? 
What  are  the  uses  of  colors?  What  is  albinism?  Where  have  you  seen  instances 
of  it?  See  Fig.  62. 

FIG.  62. 


FIG.  62.     Albino  Opossum  (Didelphys  virginiana).     Photo  by  Folsom. 

Questions  on  the  figure. — What  is  albinism  ?  In  what  structures  is  it  manifest  ? 
Among  what  groups  of  animals  can  you  find  that  it  occurs?  To  what  is  color  in 
hair  due?  What  natural  conditions  tend  to  produce  color  in  organisms?  What  is 
the  chief  value  of  color  as  an  adaptation,  individual  and  social? 

165.  Parasitism. — Of  a  nature  which  combines  the  quali- 
ties of  commensals  and  of  the  preying  animals  is  the  association 
known  as  parasitism.  It  is  an  association  of  individuals  of 
different  species  in  which  one  member  (the  parasite)  gets  all 
the  benefits,  and  the  other  (the  host)  suffers  the  loss.  It  is 
a  case  where  one  species  preys  on  another,  but  in  which  it  is 
to  the  advantage  of  the  parasite,  especially  if  a  permanent  one, 
as  well  as  of  the  host  that  the  life  of  the  latter  shall  not  be 
suddenly  destroyed.  It  will  be  readily  seen  that  the  parasite 
increases  the  work  to  be  done  by  the  host,  thus  being  a  handi- 
cap in  the  struggle  for  existence.  This  might  easily  bring 
about  the  destruction  of  these  species  which  serve  as  host  were 
it  not  for  the  fact  that  nearly  or  quite  all  animals  support  vari- 
ous parasites.  Parasites  are  of  two  classes, — external,  as  the 
fleas,  lice,  and  the  like;  or  internal,  as  most  of  the  parasitic 

9 


130  ZOOLOGY 

worms.  The  fleas  are  transient  parasites,  as  are  many  other 
insects  which  are  free  in  the  adult  stage  but  lay  their  eggs  in 
or  on  the  body  of  the  host  where  they  undergo  partial  develop- 
ment as  parasites.  In  other  instances  the  parasite  must  spend 
its  whole  life  in  the  body  of  one  or  more  hosts.  These  are 
called  permanent  parasites. 

FIG.  63. 


FIG.  63.     Caterpillar  of  Piatysamia  cecropia  parasitized.     From  Lugger. 

Questions  on  the  figure. — Seek  in  your  reference  literature  all  figures  and  refer- 
ences to  caterpillars  attacked  by  parasites.  Why  would  caterpillars  be  rather 
favorable  hosts  for  parasites?  What  are  a  few  of  the  parasitic  enemies  of  cater- 
pillars? What  economic  importance  has  this  phenomenon? 

In  addition  to  the  drain  on  the  resources  of  the  host,  the 
presence  of  the  parasite  may  so  irritate  the  tissues  of  the  host 
as  to  produce  abnormal  growth  and  disease  therein.  In  many 
of  the  transient  parasites  the  life  of  the  individual  host  is  of 
no  consequence  after  the  end  of  the  period  of  parasitism  and 
hence  the  entire  destruction  of  the  host's  body  may  occur  just 
as  truly  as  in  the  ordinary  preying  species.  Very  profound 
modifications  occur  in  the  structure  of  the  parasite,  which  are 
the  outcome  of,  and  in  part  an  adaptation  to,  the  special  mode 
of  life.  There  is  usually  a  degeneration  of  the  organs  of  di- 
gestion, of  motion,  and  of  sensation,  since  the  parasite  de- 


DIFFERENTIATION    OF    INDIVIDUALS   AND    ADAPTATION  131 

pends  on  the  host  for  the  performance  of  these  functions.  The 
explanation  of  this  degeneration  of  useless  or  unused  organs 
is  not  quite  certain.  It  is  known  that  disuse  causes  structures 
to  deteriorate  in  the  life  of  the  individual,  and  some  naturalists 
claim  that  part  of  this  IQSS  is  transmitted  to  the  next  genera- 
tion. The  claim  is  denied  by  many,  who  are  disposed  to  con- 
sider that  it  is  merely  a  case  of  natural  selection  working  for 
simplification  of  organs  and  the  economizing  of  materials. 
The  reproductive  organs  on  the  contrary  become  much  more 
complicated  and  the  reproductive  elements  are  produced  in  great 

FIG.  64. 


FIG.  64.     Lake  Lamprey  (Petromyzon  marinus  unicolor)  clinging  to  Sucker.     (From  Bull.  U.   S. 
Fish  Commission,  by  Surface.) 

Questions  on  the  figure.^Does  it  seem  that  this  is  an  instance  of  parasitism 
or  simple  preying?  What  special  organs  has  the  lamprey  adapting  it  to  this 
habit?  What  references  can  you  find  to  the  breeding  habits  of  the  lamprey? 

abundance.  This  is  an  adaptation  to  the  difficulties  involved 
in  finding  the  special  host  in  which  development  may  proceed. 
This  is  more  striking  because  many  parasites  require  two  dif- 
ferent hosts  in  order  to  complete  the  life  cycle,  and  great  mor- 
tality accompanies  the  passage  from  one  host  to  another.  A 
good  illustration  of  such  parasites  is  the  tape-worm  which 
infests  the  trout  in  Yellowstone  Lake.  The  larvae  enter  the 
tissues  of  the  trout  and  by  their  ravages  weaken  and  kill  the 
host.  The  dead  fish  are  eaten  by  pelicans.  The  worms  de- 
velop to  the  adult,  sexual  condition  in  the  digestive  canal  of 


132  ZOOLOGY 

this  second  host  and  the  eggs  or  young  embryos  escape  into 
the  water  with  the  excreta  and  from  there  are  taken  up  by 
other  trout  whose  destruction  is  again  wrought  by  the  tissue- 
infesting  larvae.  This  passage  from  one  host  to  another  prob- 
ably arose,  and  is  helped,  by  the  carnivorous  habit  among 
animals. 

The  parasites  are  almost  exclusively  invertebrates.  The 
worms  and  arthropods  furnish  the  most  numerous  representa- 
tives. The  gregarines,  and  some  other  Protozoa,  are  internal 
parasites,  often  being  parasitic  within  the  cells .  There  are  only 
a  few  parasitic  vertebrates,  and  these  are  transient.  They  be- 
long to  the  lower  fishes  (lamprey,  Fig.  64) . 

Parasitism  is  a  very  successful  adaptation  to  a  much  limited 
environment  in  which  the  organism  has  bartered  its  original 
powers  for  a  life  of  comparative  ease.  The  only  necessity 
still  resting  upon  it  is  in  the  matter  of  reproduction,  and  the 
success  with  which  this  needful  function  is  accomplished  shows 
us  that  the  parasite  must  be  considered  well  adapted  to  its 
conditions,  notwithstanding  its  degeneracy.  Its  chief  hazards 
are  met  in  the  passage  from  host  to  host  and  these  are  over- 
come by  the  carnivorous  and  omnivorous  habits  of  hosts  and 
the  extraordinary  powers  of  multiplication  on  the  part  of  the 
parasites. 

i6sa.  Practical  Exercises. — Enumerate  all  the  parasites,  transient  and  per 
manent,  known  to  infest  man,  and  find  to  what  groups  of  animals  they  belong. 
Report  on  the  habits  of  the  principal  parasites  on  man:  as  tape- worm,  trichina, 
hook-worm,  etc.  What  other  hosts  are  demanded  to  complete  the  life  cycle? 
What  are  the  principal  sanitary  conclusions  to  be  reached  ?  Examine  the  mouth- 
parts  of  the  mosquito  (see  Fig.  65).  To  what  kind  of  feeding  are  they  adapted? 

1 66.  Habits  and  Instincts  in  Relation  to  Adaptation. — In 

the  study  of  adaptations  there  is  constant  danger  lest  we 
come  to  consider  that  structures  alone  are  adaptive.  In  reality, 
adaptation  in  the  manner  of  doing  things  is  quite  as  important 
as  in  the  structure  of  the  organs  by  which  work  is  done.  When 
even  the  simplest  organisms  are  acted  on  by  an  external  stimulus 
they  respond  to  it  in  some  way.  This  response  may  be  either 
advantageous  or  disadvantageous  to  the  organism.  If  unfav- 
orable, the  result  may  be  disastrous.  If  favorable  later  repe- 


DIFFERENTIATION   OF   INDIVIDUALS   AND   ADAPTATION  133 

titions  of  the  stimulus  are  all  the  more  likely  to  be  answered 
by  the  same  kinds  of  response  as  in  the  first  instance.  This  in- 
dividual acquirement  of  a  special  mode  of  responding  to  stimuli 
is  known  as  habit.  Since  responses  in  higher  organisms  occur 
by  means  of  the  nervous  system  we  rightly  associate  habits 
with  the  nervous  activities.  In  reality,  however,  mere  pro- 
toplasm may  acquire  these  habitual  modes  of  action,  and  one 

FIG.  65. 


.m 
ma. 
1 

FIG.  65.  The  head  of  female  Mosquito  (Culex).  After  Dimmock.  a.  antennae;  c,  clypeus;  h, 
hypopharynx;  m,  mandibles;  ma.,  maxillae;  m.p.,  maxillary  palpus;  I,  labium;  la.,  labrum  (epi- 
pharynx). 

Questions  on  the  figure. — In  what  way  and  for  what  purpose  are  the  mouth- 
parts  of  the  mosquito  used?  What  are  the  probable  functions  of  the  antennae? 
Compare  the  antennas  and  the  mouth-parts  of  a  male  and  female  mosquito.  See 
also  Fig.  42.  Mention  some  respects  in  which  the  mosquito  is  adapted  to  its  mode 
of  life?  What  extent  of  horopter  do  its  eyes  command?  To  what  degree  is  the 
mosquito  parasitic? 

might  say  that  all  such  adaptations  are  dependent  on  the  power 
of  protoplasm  to  respond  to  external  stimuli.  By  reason  of 
this  power  of  adaptive  responses,  organisms  may  become 
habituated  or  acclimatized  to  changes  in  their  environment, 
their  habits  or  responses  changing  according  to  the  necessities 
of  the  case.  It  is  a  matter  of  common  observation  that  animals 
can  thus  gradually  be  brought  to  the  endurance  of  conditions 
which  would  originally  have  killed  them.  Such  must  have 
been  true  of  the  animals  which  have  come  to  live  in  the  waters 
of  hot  springs.  Such  must  have  been  the  way  in  which  other 
animals  were  changed  from  the  marine  to  the  fresh- water  habit, 
since  all  fresh-water  animals  are  believed  to  have  been  derived 
from  marine  forms. 


134  ZOOLOGY 

Similarly  in  the  history  of  any  species  those  individuals 
which  respond  in  suitable  or  advantageous  ways  to  the  stimuli 
brought  to  bear  on  them  are  selected  from  generation  to  gen- 
eration in  preference  to  those  not  so  responding,  and  in  the 
course  of  time  certain  modes  of  action  become  characteristic 
of  the  species,  even  without  the  necessity  of  individual  ex- 
perience. In  other  words  the  protoplasm  has  become  so  modi- 
fied in  a  series  of  generations  that  responses  of  a  definite  kind 
may  be  expected  of  it,  which  cannot  be  looked  upon  as  in- 
dividually acquired  habits.  These  are  instincts  and  embrace 
many  of  the  most  interesting  activities  which  have  been  men- 
tioned as  characteristic  of  animals.  The  instincts  of  feeding, 
mating,  and  the  like  are  examples.  If  instincts  are  in  conflict, 
the  stronger  prevails.  In  this  possibility  of  situations  arising 
in  which  the  instincts  are  in  conflict,  or  are  unequal  to  a  correct 
solution,  lies  the  advantage  of  intelligence  and  choice,  as  adap- 
tations whereby  correct  responses  may  be  made  to  external 
conditions.  Of  the  utmost  importance  in  the  development  of 
intelligence  is  the  introduction  of  imitation,  of  training,  of 
experience,  of  memory, — factors  more  or  less  represented  in 
the  activities  of  all  the  higher  animals.  It  is  necessary  to  re- 
member that  what  we  call  intelligence  does  not  arise  suddenly 
in  the  animal  kingdom  and  is  not  confined  to  the  highest  animals. 
Many  of  the  acts  usually  spoken  of  as  instinctive  are  not 
purely  so,  but  are  the  results,  in  part,  of  imitation,  parental 
or  social  training,  and  individual  trial  and  error,  and  are  there- 
fore to  be  classed  as  intelligent. 

167.  The  Dispersal  of  Animals  and  the  Formation  of 
Special  Faunas. — In  section  140  we  see  that  every  point  occu- 
pied by  the  individuals  of  any  species  becomes,  under  natural 
influences,  a  centre  of  distribution  from  which  the  species  will 
spread  in  all  directions,  unless  kept  back  by  adequate  barriers. 
Thus  we  should  expect  all  animals  to  be  found  all  over  the 
earth  if  all  the  conditions  were  equally  suitable  and  all  animals 
were  equally  adaptable  to  varying  conditions.  This,  however, 
is  not  so.  Species  have  unequal  powers  of  adaptation  to  the 
different  conditions  and  thus  it  comes  to  be  that  certain  groups 


DIFFEKENTIATION    OF    INDIVIDUALS    AND    ADAPTATION  135 

of  species  adapted  to  some  special  environment  will  be  found 
together  in  certain  regions,  but  will  be  absent  from  others. 
The  total  animal  life  of  any  region  is  known  as  its  fauna. 

1 68.  The  Original  Home  of  Animals,  and  the  Sea-faunas.— 
There  can  be  no  reasonable  doubt  that  animal  life  began  in  the 
sea  and  close  to  its  surface,  and  probably  not  close  to  the  shore. 
From  this  region  the  various  nooks  and  crannies  of  the  earth 
have  been  occupied,  until  now  it  seems  that  there  is  no  place 
which  does  not  have  at  least  a  few  animals  suited  to  its  condi- 
tions. The  fauna  of  the  surface  of  the  mid-ocean  is  known  as 
the  pelagic  fauna.  It  is  made  up  largely  of  protozoa;  certain 
more  or  less  transparent,  free-swimming  types  of  invertebrates, 
as  worms,  jelly-fishes,  tunicates;  many  minute  Crustacea  and 
fishes.  The  abyssal  or  deep-sea  fauna  contains  representatives 
of  all  types  of  animals  from  protozoa  to  fishes,  notwithstanding 
the  darkness  and  the  great  pressure  of  the  water.  Many  of 
the  forms  are  highly  modified,  differing  markedly  from  the 
corresponding  species  found  in  other  life-regions.  The  littoral 
or  shore  fauna  is  the  most  varied,  abundant,  and  interesting  of 
the  sea-foaunas.  Indeed  there  is  no  place  on  the  earth  where  life 
is  more  abundant.  This  is  true  because  of  the  wonderful  food 
supply  brought  from  sea  and  land  and  broken  up  by  the 
waves,  and  the  great  variety  of  physical  conditions  at  the 
meeting  of  land  and  water. 

169.  Library  Exercises. — What  are  the  special  conditions  of  each  of  the 
regions  indicated  in  the  preceding  section,  which  are  likely  to  be  favorable  or  un- 
favorable to  life?  Illustrate  more  fully  the  typical  forms  characterizing  each 
region?  Find  instances  of  the  special  adaptations  which  seem  peculiarly  advan- 
tageous to  some  of  the  animals  frequenting  each  region. 

170.  Fresh -water  Faunas. — From  the  littoral  regions  of  the 
sea,  animals  doubtless  originally  migrated  into  the  brackish 
water  of  the  mouths  of  rivers.  Thus  certain  types  came  to 
inhabit  the  fresh  waters  of  the  streams,  and  as  the  result  of 
the  adaptations  thus  made  necessary  new  species  arose  dis- 
tinctly different  from  their  relatives  which  remained  in  the 
sea.  The  most  of  the  branches  or  phyla  of  the  animal  king- 
dom have  their  fresh- water  representatives,  but  very  few 


136  ZOOLOGY 

species  of  the  sponges,  the  jelly-fish  group,  and  none  of  the 
starfish  group  have  left  the  salt  water.  Some  species  of 
animals,  as  the  salmon  and  eels,  pass  back  and  forth  from  fresh 
to  salt  water  in  obedience  to  their  spawning  or  other  instincts, 
but  these  are  not  very  numerous. 

171.  From  the  fresh-water  fauna  or  from  the  ocean  shore 
fauna  have  come  those  species  which  have  acquired  the  power 
of  breathing  by  means  of  the  air.  These  embrace  some  worms 
and  mollusks,  the  insects,  and  the  vertebrates  above  the  fishes. 
This  adaptation,  which  is  one  of  the  most  important  acquired 
in  the  history  of  animal  life  on  the  earth,  may  have  come  about 
by  the  gradual  or  periodic  drying  up  of  fresh-water  basins, 
or  by  means  of  temporary  excursions  to  the  land,  such  as  we 
see  some  water  forms  capable  of  enduring  today.  Several 
types  of  these  terrestrial  animals  have  achieved  a  more  or  less 
complete  mastery  over  the  air  (aerial  fauna)  by  means  of 
flight.  Chief  among  these  are  the  insects,  the  first  group  to 
accomplish  the  task;  a  group  of  reptiles  in  early  geological 
times;  the  birds;  and  a  few  mammals  (as  the  bats).  Animals 
after  passing  from  one  region  to  another  may  in  their  descend- 
ants reoccupy  their  old  habitat.  Thus  the  whales  and  seals  are 
air  breathing  mammals  and  are  probably  descended  from  land 
forms,  but  have  become  aquatic.  The  same  is  true  of  some 
reptiles.  Some  birds  have  lost  their  powers  of  flight  and  have 
become  purely  terrestrial. 

Other  divisions  of  the  continental  and  oceanic  faunas  into 
geographical  faunas  are  made,  depending  on  the  climatic  con- 
ditions and  the  geological  history  of  the  regions.  The  prin- 
ciples governing  this  division  are  too  complicated  for  our 
present  purposes. 

172.  Distribution  of  Animals  in  Time. — This  distribution 
of  animals  on  the  surface  of  the  earth  does  not  come  from  modern 
conditions  merely.  Throughout  all  the  millions  of  years  that  the 
earth  has  been  reaching  its  present  conditions  and  has  been 
inhabited  by  living  things,  forces  similar  to  those  we  now  know 
have  been  at  work.  During  all  this  time,  with  geographic 


DIFFERENTIATION    OF   INDIVIDUALS    AND    ADAPTATION  137 

and  climatic  changes,  the  living  things  were  changing  both  in 
their  nature  and  their  position  on  the  earth.  Thus  during 
the  various  geologic  periods,  the  distribution  and  character  of 
the  life  of  one  period  determines  the  life  of  the  next. 

All  our  knowledge  of  the  life  of  the  earlier  times  is  gained 
from  fossil  remains  found  in  the  limestone,  sandstone,  clay  and 
other  strata  of  rock.  Of  course  only  the  hard  parts  can  be 
preserved,  and  only  a  small  proportion  of  these  are  found  in  a 
form  to  give  us  much  information.  Notwithstanding,  we  are 
able  to  get  from  the  strata  a  very  fair  idea  of  the  progress  of 
life  on  the  globe.  In  the  earliest  fossil-bearing  strata  we  find 
only  invertebrate  remains.  The  invertebrates  have  continued 
through  all  the  successive  strata  to  the  present  time,  but  in 
doing  so  they  increase  in  differentiation  and  become  more  and 
more  like  present  invertebrates.  Of  the  vertebrates  the 
fishes  appeared  first,  then  the  amphibians,  reptiles,  mammals, 
and  birds.  None  of  these  when  they  first  appeared  are  like 
their  modern  types.  As  we  pass  upward  through  the  strata, 
old  species  become  extinct  and  new  ones  more  and  more  like 
the  species  of  the  present  arise  from  them,  presumably  by 
the  changes  made  necessary  in  becoming  adapted  to  the  chang- 
ing earth  conditions.  In  a  general  way  the  fossils  of  any 
age  are  intermediate— " connecting  links" — between  those  of 
the  ages  preceding  and  following.  In  other  words  plants  and 
animals  have  been  making  progress  toward  present  forms 
through  all  these  ages.  The  study  of  this  department  of 
adaptation  of  animals  is  known  as  Pal&ozoology. 

173.  Summary. 

1.  It  is  necessary  to  consider  the  individual  not  merely  as 
a  group  of  cells  and  tissues  but  as  a  unit  acting  and  being  acted 
upon  by  all  external  forces  and  by  other  organisms. 

2.  Characteristics  derived  from  the  germ  cells  of  parents, 
whether  resulting  in  similar  qualities  or  in  new  ones,  are  de- 
scribed as  hereditary.     The  reproductive  cells  are  the  carriers 
of  ancestral  qualities. 

3.  Individuals  vary  as  the  result  (i)  of  internal  conditions 
and  changes,  the  causes  of  which  are  obscure,  and  (2)  of  differ- 


138  ZOOLOGY 

ences   in   the   environment.     The   environment   may   produce 
very  important  changes  during  the  single  life  of  the  individual. 

4.  The  food  supply  of  animals  is  limited,  since  all  ultimately 
depend  on  plants;  any  species  multiplying  at  its  average  rate 
of  increase  could  in  a  short  time,  if  unchecked,  stock  the  earth 
up  to  its  limits  of  support;  that  this  does  not  occur  is  due  to  a 
struggle   for   food   among   the   excessive   numbers   which   are 
born,  whereby  only  a  small  percentage  of  them  reach  maturity. 
In  the  main,  those  survive  which  possess  some  qualities  which 
tend  to  fit  them  for  the  environment  in  which  they  find  them- 
selves.    These  are  thus  enabled  to  transmit  their  qualities  to 
their  offspring,  the  fittest   of   which   are  again   chosen.     The 
result  is   adaptation,    and  the  process   is   known   as   natural 
selection. 

5.  A  similar  result  is  effected  by  man  in  domestic  animals 
by  artificially  selecting  individuals  in  accordance  with  the  pos- 
session of  certain  features.     The  resulting  forms  are  frequently 
very  unsuited  to  the  natural  environment,  and  could  not  survive 
if  left  to  themselves. 

6.  As  the  result  of  various  causes  animals  become  dispersed 
from  their  point  of  origin,  and  in  becoming  adapted  to  the 
different   regions   into   which   they   go,    or   through   variation 
within  a  given  region,  give  rise  to  new  varieties.     When,  by 
any  means,  these  groups  have  become  perfectly  adapted  to 
their  new  special  environment  and  permanently  different  from 
their  parent  stock  and  from  each  other,  without  intermediate 
individuals  which  manifestly  connect  the  varieties,   they  are 
recognized  as  new  species.     Through  the  influence  of  heredity 
and  by  natural  selection  these   differences  may  accumulate, 
apparently  to  any  amount. 

7.  The  nutritive  function  relates  particularly  to  the  con- 
tinued existence  of  the  individual;  the  reproductive  function 
looks  to  the  continuance  of  the  species,  and  is  a  tax  on  the  in- 
dividual.    Nature   has   specially   favored   those   organisms  in 
which  an  increasing  degree  of  energy  is  given  to  the  production 
and  care  of  the  young.     As  it  is  sometimes  expressed,  nature 
sacrifices  the  individual  to  the  welfare  of  the  species. 

8.  Animals  become  adapted  to  all  the  influences  that  tend  to 


DIFFERENTIATION    OF   INDIVIDUALS   AND    ADAPTATION  139 

make  or  mar  their  success  in  life.  The  more  powerful  the 
influence  the  more  certain  the  adaptation,  because  the  destruc- 
tion is  the  more  certain  in  case  of  failure.  The  principal 
classes  of  adaptations  are, — those  relating  to  the  using  of  the 
favorable  and  resisting  the  unfavorable  features  of  the  inani- 
mate environment;  those  assisting  in  the  obtaining  of  food 
whether  vegetable  or  animal;  those  of  mating  and  care  of 
young;  those  of  offense  and  defense,  in  predaceous  animals 
and  their  prey.  The  relations  and  adaptations  range  all  the 
way  from  indifference  to  friendship,  and  from  feeding  at  the 
same  table  on  the  one  hand,  to  the  utmost  antagonism  on  the 
other. 

9.  Perhaps  the  most  important  and  the  least  understood  of 
the  series  of  adaptations  which  animals  acquire  are  those  con- 
nected with  the  nervous  system  and  its  functions: — the  habits, 
instincts,   and  intelligence  of  animals.     They  are  inseparable 
from  those  already  enumerated,  and  yet  in  fundamental  im- 
portance they  form  a  group  of  their  own.     They  seem  pri- 
marily to  depend  upon  the  irritability  of  protoplasm  which 
enables  it  not  merely  to  respond  but  to  become  permanently 
changed  by  that  response — a  kind  of  organic  memory.     From 
this  fact  acclimatization  and  adjustment  become  possible. 

10.  In  being  scattered  from  their  starting  place,   animals 
with  similar  powers  of  response  and  adaptation  come  to  be 
located  in  the  same  kinds  of  conditions.     This  results  in  faunas 
more  or  less  characteristic  of  all  the  important  kinds  of  en- 
vironments: as  marine,  brackish  water,  fresh  water,  terrestrial, 
aerial,  cavern  faunas,  etc. 

11.  The  origin  of  animal  life  was  in  the  ocean,  and  from 
these  marine  types  it  is  believed  that  all  other  forms  of  animal 
life  have  come,  by  gradual  adaptation  to  their  present  mode 
of  life. 

12.  The  various  climatic  zones  of  the  earth  and  the  principal 
geographical   regions   are   characterized   by   distinct   forms   of 
life.     For  example,  the  lake  life  of  Africa  differs  from  that  of 
North   America,    and   similarly   for   all   the   various   types   of 
fauna.     An  analysis  of  such  facts  and  an  explanation  of  them 
belongs  to  the  geographical  distribution  of  animals. 


140  ZOOLOGY 

13.  Fossils  are  remains  of  former  plant  and  animal  life 
preserved  in  the  rocks.  We  read  much  of  the  ancient  life 
history  by  a  study  of  these  fossils.  We  learn  chiefly  that  life 
started  in  a  more  lowly  and  more  simple  form  than  we  now  find 
it;  that  it  has  been  getting  more  complex  and  more  like  the 
present  with  each  passing  age.  What  we  find  then  at  the  present 
moment  on  the  earth  is  not  the  result  of  present  forces.  It  is 
the  result  of  all  the  past. 

174.  Topics  for  investigation,  in  field,  laboratory  and  library: 

1 .  What  constitutes  individuality  in  animals  ? 

2.  In  what  respects  (enumerate)  and  to  what  degree  have  you  ever  noticed 
variety  in  a  given  species?     In  the  offspring  of  a  pair  of  parents? 

3.  Have  you  ever  observed  any  changes  in  structure  in  animals  which  could 
reasonably  be  attributed  to  change  in  environment  ?     Give  evidence. 

4.  Does  use  or  disuse  produce  changes  in  the  organs  of  an  individual?     Why? 
Give  illustrations. 

5.  Enumerate  some  facts  of  your  own  observation  which  illustrate  heredity. 

6.  Cite  observed  instances  of  associations  among  animals  of  the  same  species, 
and  determine  as  well  as  you  can  from  your  observations  what  ends  are  gained  by 
the  association. 

7.  Make  an  effort  to  classify  a  series  of  objects,  noting  carefully  your  basis  of 
classification;  that  is,  the  characters  which  you  select  in  separating  and  grouping 
the  individuals.     The  teacher  can  make  this  a  most  instructive  exercise.    A  few 
objects  of  considerable  diversity  may  be  chosen,  as  sand,  pebbles,  shells,  crystals, 
a  plant,  an  animal,  and  the  student  may  be  required  to  examine  each  as  fully  as 
he  can,  write  out  the  characters  which  he  discovers  as  belonging  to  each,  being 
sure  that  he  uses  a  simple  and  observed  feature  in  each  statement.     On  the  basis  of 
these  recorded  observations  let  him  compare  and  group  the  objects.     Or  take  a 
large  number  of  relatively  similar  individuals  and,  without  stopping  to  write 
their  characters,  let  the  student  place  or  distribute  them  in  groups  near  or  remote 
from  each  other  in  proportion  to  their  unlikenesses,  allowing  intermediate  forms 
to  stand  between.     Afterward  he  may  be  caused  to  determine  and  justify  his  classi- 
fication and  to  see  whether  other  classification  could  be  made  with  a  different  basis. 
Gasteropod  shells,  illustrating  varieties  of  the  same  and  different  species;  beetles; 
butterflies;  grass-hoppers;  or  even  books  of  diverse  shape,  binding  and  contents 
may  be  used. 

8.  Can  you  suggest  any  cause  for  the  degeneracy  of  parasites? 

9.  Cite  instances  from  you  own  observation  in  which  animals  use  the  leap  or 
spring  in  capturing  prey  or  escaping  enemies.     Why  is  it  a  peculiarly  advantageous 
adaptation  ? 

10.  Cite  instances  of  the  food-storing  instinct,  with  all  observed  details.     What 
is  the  most  remarkable  fact  about  them?     How  is  it  useful? 

11.  From  reading  and  observation  would  you  say  that  there  is  any  definite 
relation  between  the  instinct  for  home-building  and  parental  care? 

12.  Study  sleep  among  animals.     What  is  its  relation  to  rest?     Is  it  found  in 
the  lower  animals  ?     To  what  is  sleep  an  adaptation  ?     When  does  sleep  commonly 
occur  among  animals ?     Why?     Do  plants  show  any  sleeping  qualities? 


DIFFERENTIATION   OF   INDIVIDUALS   AND   ADAPTATION  141 

13.  What   are   the   principal  geographical  faunas  recognized  by  zoologists. 
Enumerate  the  more  important  means  by  which  dispersal  of  animals  from  one 
region  to  another  occurs.     What  are  the  chief  barriers  to  the  dispersal  of  land 
animals?  of  aquatic  animals? 

14.  Study  the  different  authors  to  which  you  have  access,  as  to  the  significance 
of  the  terms  species  and  variety  (or  sub-species). 

15.  What  were  the  older  views  concerning  the  "fixity"  or  invariability  of 
species? 

1 6.  What  are  the  different  views  of  the  "Origin  of  Species,"  as  based  on  the 
views  of  the  meaning  of  species  ? 

17.  What  is  the  essential  difference  between  the  theories  of  "natural  selection" 
and  "definite  variation"  as  explaining  adaptation  of  organisms  to  their  environ- 
ment. 

1 8.  What  additional  ideas  are  introduced  by  the  "mutation"  theory  of  De 
Vries?     What  reasons  does  he  give  for  thinking  that  natural  selection  is  not  a  very 
important  factor  in  adaptation  ? 


CHAPTER  IX 

A  GENERAL  REVIEW  OF  THE  ANIMAL  KINGDOM 

Before  undertaking  the  study  of  the  special  groups  into 
which  animals  are  arranged  because  of  their  apparent  kinships, 
it  will  be  advantageous  for  the  student  to  look  briefly  at  the 
whole  field  of  animals, — the  "animal  kingdom."  See  Fig.  66. 

175.  Class  Mammals. — Beginning  with  man  himself  it  is  easy 
to  see  that  there  are  numerous  animals  (as  the  apes  and  monkeys; 
the  various  quadrupeds,  as  the  horse,  ox,  dog,  cat,  bears  and 
squirrels;  the  whales  and  seals;  and  many  others)  which  differ 
much  in  general  appearance  from  him  but  are  like  him  in  very 
many  remarkable   particulars.     For  example,   they   all  bring 
forth  their  young  alive  and  in  a  more  mature  condition  than 
is  usual  for  other  types  of  animals,  the  young  being  carried 
in  a  special  organ  of  the  mother's  body,  often  until  develop- 
ment is  well  advanced.     After  birth  the  mother  produces  milk 
in  special  glands  for  the  nourishment  of  the  young  to  a  still 
more  mature  stage.     This  is  seen  in  no  other  group  of  animals 
beside  the  mammals.     The  skin  produces  hair  or  wool  as  a 
covering  for  the  body.     Man  differs  from  the  other  mammals 
in  certain  particulars  but  not  nearly  so  much  as  he  and  they 
differ  from  other  animals. 

176.  Class  Birds. — Another  well-developed  and  numerous 
group  of  animals  is  the  class  known  as  birds.     There  is  scarcely 
another  class  of  animals  so  easy  to  distinguish  at  sight  as  this. 
They  equal  or  surpass  the-  mammals  in  specialization,  but  are 
very  different  from  them.     They  are  especially  to  be  recognized 
by  the  body-covering  of  feathers,  the  modification  of  the  front 
limbs  into  wings  for  purposes  of  flight,  and  the  fact  that  the 
jaws  are  sheathed  in  horny  matter  and,  at  least  in  present  day, 
birds  do  not  possess  teeth. 

177.  Class  Reptiles. — This  is  a  class  recognized  by  zoologists 
which  is  not  nearly  so  easy  to  define  or  to  identify  as  either  of 

142 


A   GENERAL   REVIEW   OF   THE   ANIMAL  KINGDOM  143 

the  preceding.  This  is  partly  because  the  animals  composing 
it  differ  more  among  themselves  than  in  the  other  classes.  It 
includes  snakes,  lizards,  turtles,  and  crocodiles.  The  reptiles 

FIG.  66. 


REPTILES 

5500 


POaiFERA 

2500 


FIG.  66.  Diagram  showing  the  general  relations  of  the  chief  divisions  of  the  animal  kingdom. 
The  number  of  species  belonging  to  each  is  roughly  approximate,  only.  During  the  last  30  years 
the  number  of  species  of  animals  recognized  by  zoologists  has  increased  at  the  average  rate  of 
12,000  per  year. 

have  some  features  which  indicate  that  they  may  be  distantly 
related  to  both  birds  and  mammals,  as  well  as  to  the  next 
class.  This  is  an  additional  reason  why  the  group  of  reptiles 
is  a  difficult  one  to  define.  In  general  they  may  be  recognized 


144  ZOOLOGY 

by  the  fact  that  their  bodies  are  covered  by  scales  or  plates 
instead  of  hair  or  feathers.  They  always  breathe  oxygen  from 
the  air,  as  do  birds  and  mammals.  They  usually  have  only 
three  chambers  to  the  heart  whereas  in  the  former  groups 
there  are  four.  The  blood  is  not  constantly  warm  as  in  birds 
and  mammals.  They  lay  eggs  very  much  like  those  of  birds. 

178.  Class  Amphibians. — In  external  appearance  the  mem- 
bers of  this  class  often  look  somewhat  like  reptiles,  and  they 
have  certain  possessions  in  common  with  them,  as  the  cold  blood 
and  the  three-chambered  heart.    They  are  especially  noteworthy 
from  the  fact  that  they  begin  life  breathing  oxygen  from  the 
water  as  fishes  do,  and  later  in  life  lose  their  gills,  acquire 
lungs,  and  get  their  oxygen  from  the  air,  as  do  the  reptiles 
and  higher  forms.     Amphibians  include  the  frogs,  toads  and 
salamanders.     This  is  not  a  very  important  group  in  nature, 
but  is  intensely  interesting  to  the  student  of  zoology  because 
it  seems  to  be  a  connecting  link  between  the  air-breathing  and 
the  water-breathing  forms. 

179.  Class  Fishes. — Fishes  are  characterized  by  the  fact 
that  they  breathe  by  means  of  gills  throughout  life.     The  body 
is  often  scaly;  the  appendages  are  fin-like;  the  blood  is  cold, 
and  the  heart  has  two  chambers.     They  are  beautifully  adapted 
to  life  in  the  water  and  are  easily  recognized. 

1 80.  Vertebrates   and   Invertebrates. — All  the   animals   of 
which  we  have  thus  far  spoken  agree  in  certain  particulars. 
They  all  possess  a  dorsal  rod  of  supporting  matter  (notockord; 
see  §349),  which  is  often  surrounded  by  cartilage  or  bone  (the 
vertebral  column).     The  nervous  system  in  all  of  them  is  dorsal 
to  this  rod  and  to  the  digestive  tract,  and  is  tubular  in  char- 
acter.    The  heart  is  ventral  to  the  digestive  tract  and  the 
blood  has  red  corpuscles.     They  are  called  Chordata  (having 
notochord)  or  Vertebrata.     This  is  the  highest,  best  developed 
phylum  of  the  animal  kingdom.     The  five  classes  which  have 
been  mentioned  are  included  in  it.     All  other  animals,  with  the 
exception  of  a  few  which  seem  intermediate  in  some  respects, 
are  classed  as  Invertebrates,  and  agree  in  general  in  the  follow- 


A    GENERAL   REVIEW    OF    THE    ANIMAL    KINGDOM  145 

ing  facts: — there  is  no  notochord  or  vertebral  column;  the 
nervous  system  is  chiefly  ventral  to  the  digestive  tract;  the 
heart,  when  present  is  dorsal;  and  the  blood  usually  has  only 
colorless  cells.  The  principal  phyla  of  the  Invertebrates 
follow. 

181.  Phylum  Arthropoda   (jointed  feet). — This  is  the  most 
numerous    phylum    of    the    animal    kingdom.     It    embraces 
crayfish,  lobsters,  crabs  (Crustacea),  which  for  the  most  part 
have  gills  and  live  in  water;  the  Insects,  as  bees,  flies,  beetles, 
butterflies,  etc.,  which  usually  live  in  the  air  and  get  their 
oxygen  from  it;  the  spiders,  whose  habits  and  appearance  are 
somewhat   similar   to   those   of   the   insects.     Arthropods   are 
especially  to  be  recognized  by  the  fact  that  their  bodies  are 
segmented,  are  bilaterally  symmetrical,  and  have  paired  jointed 
appendages  to  many  of  the  segments.     In  addition  to  this  there 
is  a  covering  of  resistant  substance  (chitin)  developed  by  the 
skin.     This  serves  for  the  protection  of  the  animal  and  for  the 
attachment  of  the  muscles  within. 

182.  Phylum  Mollusca  (soft). — This  branch  of  the  inverte- 
brates includes  the  snail,  clam  and  oyster,  the  squid  and  devil- 
fish, and  their  kind.     They  differ  very  much  among  themselves 
but  agree  in  the  lack  of  segmentation  of  their  bodies,  in  the 
absence    of    paired    appendages, — and    in    those    types    most 
commonly  known  to  the  student,  in  the  presence  of  a  shell 
of  one  or  two  valves,  which  is  secreted  by  a  fold  of  the  skin 
called  the  mantle.     While  many  of  the  mollusks  are  lowly  in 
organization  and  in  intelligence,  one  group  of  them — that  which 
includes    the    squid,— has    the    most    highly    developed    brain 
found  below  the  vertebrates.     It  occupies  among  the  inverte-, 
brates  somewhat  the  place  which  man  has  among  the  mammals. 

183.  Phylum  Echinodermata  (spiny  skin). — These  are  easily 
recognized  by  the  possession  of  five  or  more  arms  or  rays  in 
the  adult  stage'.     Usually  a  skeleton  is  developed  in  the  skin. 
This  is  often  covered  with  spines,  and  from  this  fact  the  phylum 
has  its  name.     They  are  marine  and  are  poor  movers, — a  few 
being  fixed  by  stalks  to  objects  in  the  ocean.     The  starfish, 
sea-urchin  and  sea-lilies  are  representatives. 


146  ZOOLOGY 

184.  Phylum  Annulata  (Segmented  Worms:  with  rings).— 
This  phylum  is  similar  to  the  arthropods  in  that  the  body  is 
bilaterally  symmetrical,  is  segmented,  and  has  paired  append- 
ages to  many  of  the  segments.     It  differs  from  them  in  the 
fact  that  the  appendages,  when  present,  are  not  jointed  but 
are  merely  setae  or  hairs  in  sockets  or  on  fleshy  prominences. 
The  segments  are  more  nearly  homonomous   than   in  typical 
arthropods.     The  earthworm,  many   types  of  aquatic  worms, 
and  leeches  are  included  here. 

185.  Unsegmented    Worms    (embracing    numerous    ill-as- 
sorted animals   of   doubtful   relationship).     Here  may   be  in- 
cluded a  number  of  small  groups  many  of  which  have  long 
been  grouped  with  the  Annulata  and  called  "worms."     They 
are  not  sufficiently  alike  to  be  regarded  as  one  distinct  phylum; 
Indeed  there  are  probably  three  or  four  small  phyla  included. 
The  majority  of  them  are  bilaterally  symmetrical,  unSegmented 
and  without  appendages.     They  differ  from  the   mollusks  in 
that  they  do  not  possess  a  mantle  and  do  not  secrete  a  shell. 
Many  of  them  are  parasitic.     Among  these  animals  of  doubtful 
relationship  may  be  included  the  "flat- worms,"  "thread- worms," 
the  nemertea,  rotifers,  and  others. 

1 86.  Ccelomata     (with    ccdom)    and    Ccelenterata     (hollow 
inside). — All  the  animals  thus  far  considered  possess  during 
some  stage  of  life  a  more  or  less  developed  body  cavity  or 
ccelom   (see   §58)   distinct  from  the  digestive  tract.     For  this 
reason  they  are  sometimes  known  collectively  as   Ccelomata. 
All  the  remaining  many- celled  animals  have  a  general  cavity 
which  serves  both  as  a  body  cavity  and  a  digestive  tract  (gastro- 
vascular  cavity), — or  to  speak  more  exactly,  there  is  no  true 
body   cavity.     Of  these  the   phylum  Ccelenterata  is  the  chief 
illustration.     Here  belong  the  jelly-fish,   sea-anemone,   corals. 
They  are  all  aquatic  and  are  more  or  less  tubular,  sac-shaped 
animals  often  attached  by  one  end,  with  the  mouth,  which  also 
functions  as  the  anus,  at  the  other  surrounded  by  clusters  of 
tentacles.     Many  secrete  skeletons,   and  some  form  immense 
attached  colonies. 


A   GENERAL   REVIEW    OF    THE    ANIMAL    KINGDOM  147 

187.  Phylum  Porifera  (pore-bearing). — This  group,  to  which 
belong  the  sponges,  is  sometimes  classed  with  the  Ccelenterata. 
While  similar  to  them  in  habit  the  sponges  are  much  less  highly 
organized  and  unified.     Instead  of  a  single  mouth  opening  into 
the   digestive   tract,    sponges   have   many   openings   or   pores 
(whence  the  name  Porifera)  which  are  the  beginnings  of  tubes 
entering  a  central  cloaca  or  sewer.     This  is  in  reality  not  a  true 
digestive  tract.     It  communicates  with  the  exterior  by  one  or 
more  large  passages.     They  are  attached  and  usually  form  large 
colonies  by  budding. 

1 88.  Phylum  Protozoa   (first  animals). — All  the  preceding 
phyla  of  animals  consist,  in  the  adult  stage,  of  many  cells  among 
which  there  is  more  or  less  differentiation.     In  all  of  them  the 
adult  passes  through  stages  in  which  the  cells  are  arranged  in  at 
least  two  layers   (ectoderm  and  entoderm;  see  §55),  from  which 
the  tissue-masses  arise.     These  animals  are  known  as  Metazoa. 
In    the    remaining    phylum — the    Protozoa — the    animals  •  are 
single   cells,    or   at   most,    loose   aggregations   of   similar   cells. 
They  are  the  lowest  of  animals  and  are  for  the  most  part  in- 
visible to  the  naked  eye. 

189.  An  Artificial  Key  to  the  Phyla  of  the  Animal  Kingdom. 

Many-celled   animals       METAZOA. 

With  true  coelom     Coelomata. 

Possessing  notochord  (and  often  vertebral  column),  • 

Phylum  Chordata. 
Possess  functional  gills. 

Throughout  life     Class  Fishes. 

In  embryonic  life  only  (with  a  few  exceptions), 

Class  Amphibia. 
Do  not  possess  functional  gills. 

Epidermal  covering  of  scales Class  Reptiles. 

Epidermal  covering  of  feathers    Class  Birds. 

Epidermal  covering  of  hair Class  Mammals. 

Without  notochord* Invertebrata. 

Bilaterally  symmetrical  (chiefly). 
Body  made  up  of  segments. 

Paired  appendages  jointed Phylum  Arthropoda. 

Paired  appendages  unjointed Phylum  Annulata.  _ 

Body  unsegmented;  without  paired  appendages. 
With  mantle — often  secreting  shell, 

Phylum  Mollusca. 
No  mantle Unsegmented  Worms. 


148  ZOOLOGY 

Radially   symmetrical   in   adult Phylum   Echinodermata. 

Without  true  coelom. 

With  a  single  mouth,  which  also  functions  as  an  anus:  stinging  cells 

Phylum  Ccslenterata. 
With  numerous  incurrent  openings  or  pores,  and  only  one — or  few — 

excurrent.     No  stinging  cells Phylum  Porifera. 

Single-celled  animals   (chiefly) Phylum  Protozoa. 


CHAPTER  X 

PHYLUM  I.— PROTOZOA  (Primitive  Animals) 

LABORATORY  EXERCISES 

Without  compound  microscopes  this  branch  of  animals  can- 
not be  studied  with  profit  in  the  laboratory.  The  Amoeba  is 
one  of  the  most  interesting  of  the  Protozoa  and  serves  well  to 
illustrate  the  simplest  forms  of  animal  life,  but  large  specimens 
in  sufficient  numbers  for  profitable  study  in  an  elementary  class 
are  usually  so  difficult  to  secure  at  the  right  time  that  it  be- 
comes a  question  whether  the  teacher  should  be  advised  to 
depend  on  them.  My  advice  is,  make  every  arrangement  you 
can  to  secure  them,  use  them  for  demonstration  or  study 
whenever  they  appear,  but  depend  on  Paramecium.  Perhaps 
the  surest  method  for  securing  Amoeba  is  to  chop  up  the  soft 
parts  of  three  or  four  fresh-water  mussels,  placing  the  pieces, 
together  with  the  shells,  in  a  large  shallow  basin.  Allow  a 
gentle  stream  of  water  to  drip  into  this.  This  keeps  the  water 
slightly  agitated,  causes  it  to  run  over,  and  prevents  an  undue 
accumulation  of  bacteria.  The  addition  of  a  little  of  the  sur- 
face mud  secured  from  the  bottom  of  several  streams  or  ponds 
will  make  the  success  of  the  preparation  all  the  surer.  Amcebas 
should  appear  at  the  surface  of  the  mud,  about  the  shells, 
or  at  the  margins  of  the  vessel  near  the  surface  of  the  water. 
Test  all  these  places  every  day,  and  sooner  or  later  the  amoebas 
are  practically  sure  to  be  found.  Paramecia  will  be  likely 
to  occur  in  the  same  preparation.  Any  abundant  Protozoan 
which  may  appear  may  be  studied  instead  of  Paramecium  or 
in  addition  to  it,  by  means  of  the  outline  below.  The  mode 
of  securing  the  materials  should  be  explained  to  the  class  to 
make  clearer  the  habits  of  these  organisms. 

190.  Paramecium. — This  protozoan  may  be  obtained 
readily  by  allowing  fresh- water  algae,  with  hay  or  leaves,  to 

149 


150  ZOOLOGY 

decay  in  water.  This  infusion  should  be  examined  every  day. 
If  the  bacteria  become  too  abundant  some  of  the  surface  water 
may  be  poured  off  and  fresh  water  added.  The  paramecia, 
which  are  just  visible  to  the  naked  eye,  appear  as  a  whitish 
cloud  in  the  water  or  may  accumulate  as  a  film  at  the  surface. 
Often  a  sufficient  number  for  study  may  be  secured  by  scraping 
with  a  scalpel  the  matter  which  accumulates  on  the  sides  of  the 
vessel  just  beneath  the  water  surface,  even  when  they  are  not 
sufficiently  numerous  to  cloud  the  infusion.  The  cover-glass 
should  be  supported  by  sediment  or  by  bits  of  cover-glass. 
Make  outline  sketches  of  everything  which  can  be  shown  in 
that  way. 

I.  With  the  low  power  of  the  microscope  study  the  follow- 
ing points: 

1.  Activities. — Describe,  and  figure  as  well  as  possible,  the 
nature  of  all  the  movements  of  which  the  animal  seems  capable, 
using  arrows  to  indicate  directions.     Can  you  distinguish  an 
anterior  from  a  posterior  end?     By  what  characteristics? 

Do  you  find  any  reasons  for  believing  that  the  paramecia 
are  sensitive  to  external  influences?  What  evidences?  To 
what  sorts  of  influences  do  they  respond?  Do  they  avoid  ob- 
jects? Do  they  collide  with  each  other  in  motion?  Do  they 
tend  to  collect  ?  Where  ?  Are  they  as  active  at  the  end  of  the 
hour  as  at  the  beginning  ? 

Make  a  new  preparation  in  which  the  paramecia  are  uni- 
formly distributed  in  a  drop  of  water.  Place  a  very  small 
grain  of  salt  at  the  edge  of  the  drop.  What  is  the  result? 
Watch  the  individuals  under  the  microscope  as  they  come  into 
the  salt  solution.  On  a  new  preparation,  try  similarly  a  minute 
amount  of  acetic  acid  (Ko  to  %  per  cent,  solution)  applied  with 
a  capillary  tube.  Compare  results.  Try  sugar;  quinine. 

Do  you  discover  any  instances  of  division  or  conjugation? 
If  so,  describe. 

2.  General  form  of  the   body.     How   would  you   describe 
its  shape?     To  what  degree  is  it  capable  of  change?     Is  the 
body  symmetrical  ?     Give  evidences.     Make  diagrams  showing 
your  idea  of  a  cross-section  through  the  middle;  also  of  one, 
one-third  way  from  each  end. 


PROTOZOA  151 

II.  With  the  high  power,  study, — 

3.  Cilia:  where  found?     Are  they  uniform  in  length?     How  do  they  act? 
What  results  do  they  produce?     (Place  a  small  amount  of  water  containing  finely 
powdered  indigo  or  carmine  at  edge  of  cover-glass.     If  the  movements  are  too 
rapid  a  little  gelatine  added  to  the  water  will  be  of  advantage.) 

4.  Find  the  mouth,  with  the  oral  groove  leading  to  it.     Position  and  shape? 
How  are  food  particles  captured?     Can  you  find  them  within  the  body  (food 
vacuoles)  ?     How  are  the  food  vacuoles  formed  ?     Do  the  food  vacuoles  move  within 
the  cell?     If  so,  trace  their  course?     What  finally  becomes  of  them?     Evidences? 

5.  Contractile  vacuoles  (clear  spherical  objects  rhythmically  disappearing  and 
reappearing).     Number?     Position?     Rate  of  contraction?     Do  they  contract 
at  the  same  time?     What  becomes  of  the  clear  material  during  the  contraction  of 
the  vacuole?     Are  they  deep  or  superficial  structures?     Your  evidences?     Does 
change  of  temperature  cause  any  change  in  their  rate  of  contraction? 

6.  Distinguish  between  the  inner  mass  of  protoplasm  (endosarc)  and  an  outer 
layer  (ectosarc).     What  are  the  characteristics  of  each  as  regards  motion,  clearness, 
firmness,  etc.  ?    Note  the  changes  in  these  portions  on  the  addition  of  dilute  acetic 
acid  or  iodine  at  the  edge  of  the  cover-glass. 

7.  Discover,  if  possible,  nuclear  bodies.     These  are  not  usually  recognizable 
without  careful  staining.     Place  at  the  edge  of  the  cover-glass,  in  a  fresh  preparation 
of  Paramecia,  a  5-10%  aqueous  solution  of  methyl  green.     Compare  the  result 
with  a  permanent  mount  stained  by  suitable  methods  (see  Appendix :  Suggestions 
for  the  Laboratory). 

191.  Other  Protozoa. — If  the  class  is  supplied  with  microscopes,  the  pupils 
should  be  allowed  to  examine  stagnant  water  for  as  many  types  of  protozoa  as 
may  be  found.  Allow  them  to  compare  these,  noting  the  points  of  similarity  and 
difference  in  general  structure  and  activities.  Especially  profitable  protozoa  for 
laboratory  work  are  the  green  flagellate  infusorian,  Euglena,  which  often  tinges  the 
water,  or  forms  a  green  scum  over  shallow  pools  of  water;  the  colonial  ciliate  form, 
Vorticella,  found  attached  to  submerged  objects  in  ponds  or  pools  of  slowly  moving 
streams  in  which  there  is  considerable  decaying  organic  matter.  The  colonies  are 
easily  visible  to  the  naked  eye.  Stentor  is  a  very  large  trumpet-shaped  infusorian 
which  may  be  alternately  attached  and  free-swimming.  It  lives  upon  submerged 
sticks  and  leaves  and  may  often  be  found  attached  to  the  sides  of  vessels  in  which 
such  matter  has  been  placed.  In  all  such  studies  and  identification  of  the  protozoa 
the  question  of  evidence  of  the  unicellular  character  of  the  organism  should  be  kept 
before  the  student. 

DESCRIPTIVE  TEXT 

192.  In  this  first  and  lowest  group  of  animals,  the  individu- 
als of  which  consist  of  single  cells  or  loosely  associated  simi- 
lar cells,  we  find  something  of  the  variety  of  shape  which  we 
observed  in  the  tissue  cells  of  the  higher  animals  (Chapter  V). 
The  Protozoa  are  especially  interesting  to  the  biologist  because 
they  represent  the  simplest  forms  of  animal  life  now  found 
on  the  earth  and  because  some  of  their  representatives  are  very 


152  ZOOLOGY 

like  some  of  the  simplest  plants.  Indeed  some  of  them  are 
claimed  by  both  the  botanists  and  the  zoologists.  It  also  seems 
probable  that  the  first  animal  life  to  appear  on  the  globe  had 
the  general  characteristics  of  some  of  the  Protozoa.  Whether 
some  type  of  protozoan  is  to  be  considered  as  the  ancestor  of 
the  higher  many-celled  animals  or  not,  it  is  true  that  we  find 
illustrated  here  in  the  simplest  possible  way  the  beginning  of 
all  those  functions .  which  are  so  completely  distributed  among 
the  special  organs  of  the  complex  animals.  The  Paramecium 
does  in  a  simple  yet  satisfactory  way  all  that  any  complex  living 
animal  needs  to  do  in  order  to  live  and  perpetuate  its  species. 


FIG.  67.     Amoeba,     ec.,  ectosarc;  en.,  endosarc,  containing  food  vacuoles  (/);  n,  nucleus;  p,  pseudo- 
podium;  p.v.,  pulsating  vacuole. 

Questions  on  the  figure. — Define  the  various  terms  used  above  in  describing 
the  parts  of  the  amoeba.  What  changes  may  the  amoeba  undergo  in  its  life  history  ? 
Compare  with  figures  2  and  7. 

193.  General  Characters. 

1.  Mostly  unicellular  throughout  life.     May  have  one  or 
more  nuclei  (Figs.  68-71). 

2.  The  protoplasm  usually  consists  of  a  clearer  outer  por- 
tion (ectosarc)  and  a  more  granular  inside  portion  (endosarc) 
(Fig.  68,  ec,  en). 

3.  There  is  usually  what  is  known  as  a  pulsating  vacuole, 
in  which  some  of  the  more  fluid   cell-contents   collect,  to  be 
forced  out  of  the  vacuole  again  by  the  contraction  of  the  denser 
protoplasm  (Fig.  68,  pv). 


PROTOZOA  153 

4.  Reproduction  is  effected  chiefly  by  dividing  into  two  or 
more  parts  or  cells,  which  occasionally  remain  associated.  The 
nucleus,  when  present,  divides  with  the  division  of  the  cell 
(Fig.  7). 

FIG.  68. 


t.c 
t.n. 


FIG.  68.  Paramecium  in  optical  section  (semi-diagrammatic).  A,  anterior  end;  c,  cilia;  t.c., 
ectosarc;  e.n.,  endosarc;  /.».,  food  "vacuole";  g,  gullet;  N,  meganucleus;  n,  micronucleus;  o,  oral 
groove,  leading  to  the  mouth;  p.v.,  pulsating  vacuoles  in  different  stages  of  contraction;  tr.,  tricho- 
cysts;  v,  food  vacuole  in  process  of  formation. 

Questions  on  the  figure. — In  what  sense  is  the  term  "vacuole"  descriptive  of 
the  structures  to  which  it  is  applied  in  Paramecium?  Describe  the  special  adapta- 
tions of  the  anterior  end.  Judging  from  their  distribution  have  the  cilia  any  other 
function  than  locomotion?  In  what  way  are  the  food  vacuoles  formed?  Why  do 
some  food  vacuoles  appear  lighter  than  others? 

194.  Habitat. — Protozoa    in    their    active    stages    require 
abundant  moisture,  hence  they  are  found  in  water,  fresh  or 
salt,   and  as  parasites  in  the  bodies  of  other,  animals.     The 
Sporozoa  are  parasitic.     Some   amoeboid  Rhizopods  infest  the 
digestive  tract  of  man  and  other  animals,  producing  irritation 
and  disease.     The  Infusoria  occur  in  water  in  which  there  is 
decaying   organic   matter   and   minute   organisms   of   various 
kinds.     Vohox    and   Euglena,    green    forms    often    classed    as 
Protozoa,  have  the  power  which  green  plants  possess  of  using 
the  inorganic  substances  found  in  ordinary  water  in  building 
up  their  substance. 

195.  Organization. — We   cannot   say   that    Protozoa   have 
organs  in  the  sense  in  which  we  have  defined  that  term  hitherto, 
yet   they   are    certainly   organized.     The   organization   shows 
itself  in  the  nucleus,  in  the  distinction  of  ectosarc  and  endosarc, 


154  ZOOLOGY 

in  the  pulsating  and  food  vacuoles,  in  temporary  projections 
of  protoplasm  called  pseudopodia,  in  more  permanent  vibratile 
projections  of  the  ectosarc  known  as  cilia  or  flagella,  in  the 
mouth — found  in  many  forms,  in  cell- wall  and  secreted  skele- 
ton, in  delicate  contractile  fibres  in  the  ectosarc,  and  in  stalks 
for  attachment  to  objects  (see  Figs.  68  and  70).  By  means 
of  these  differentiations  all  the  functions  necessary  to  life  are 
performed.  There  are  many  colonial  Protozoa.  In  such  (as 
Volvox)  there  may  be  some  division  of  labor  among  the  cells, 
— as  between  reproductive  cells  and  body  cells  (Figs.  72,  73). 

196.  Nutrition. — The   parasites    absorb    food,    already    di- 
gested  and  fitted  for  absorption,    directly  from  their  hosts. 
Most  of  the  free  forms  take  solid  particles  directly  into  the 
endosarc   through   permanent   or   temporary   openings  in   the 
ectosarc.     In  some  shelled  forms,  in  which  there  is  no  mouth, 
the  food  is  digested  outside  the  body  proper  (Fig.  74)  by  the 
pseudopodia.     These  envelop  the  food  and  gradually  transfer 
it  to  the  main  body  of  protoplasm  through  openings  in  the 
shell.     In   the   other   instances   the   digestion   takes   place   in 
the  body  of  the  protoplasm.     The  ferments  found  in  the  proto- 
plasm are  doubtless  responsible  for  the  digestive  changes  and 
act  in  much  the  same  way  as  the  special  ferments  secreted 
from  the  cells  of  the  digestive  glands  in  the  higher  animals. 
Circulation  is   effected  'by   the   general  protoplasmic  motion. 
Respiration,    whereby   the   protoplasm   gets   rid   of   C02   and 
receives   O,   occurs   through   the   cell   surface   without   special 
structures.     All    projections    of    the    cell-body    assist    in    this 
exchange   by   increasing   the   area   of  the   surface.     Excretion 
may  take  place  from  the  surface  of  the  cell,  and  it  seems  probable 
that  the  contractile  vacuole  has  an  excretory  function. 

197.  Movement. — The  majority  of   Protozoa  move   freely 
in  their  medium.     In  Amoeba  the  motion  is  of  a  gliding  character 
and  is  effected  by  putting  forth  processes  into  which  the  proto- 
plasm streams.     The  process  or  pseudopodium  thus  enlarges  at 
the  expense  of  the  body  of  the  cell  and  progress  is  had  in  the 
direction    of    the    growing    pseudopodium.     The    direction    of 
motion  is  changed  by  the  breaking  out  of  new  processes  in  a  new 


PROTOZOA  155 

direction.  In  those  Protozoa  which  have  a  cell-wall  special 
devices  become  necessary  to  enable  the  animals  to  move.  Most 
of  the  free-swimming  forms  possess  cilia  or  flagella,  which 
act  as  oars  on  the  water  and  thus  propel  them.  In  Stentor, 
Spirostomum,  Vorticella,  etc.,  there  are  clearly  defined  strands 
of  contractile  material  developed  in  the  ectosarc  by  which  the 
shape  of  the  animal  may  be  strikingly  changed.  In  the  at- 
tached forms  these  strands  extend  from  the  body  proper  into 
the  stalk.  Vorticella  (Fig.  70)  by  this  device  may  change  its 
position  with  much  suddenness.  Attached  forms  are  able  to 
break  loose  from  their  moorings  and  become  free-swimming 
for  a  time.  Still  other  species  are  encased  in  shells  and  are 
almost  or  wholly  destitute  of  the  power  of  independent  motion. 
Even  the  most  active  types  may  assume  the  non-motile  or 
resting  stage,  by  which  they  pass  uninjured  through  such 
unfavorable  conditions  as  drouth,  cold,  and  the  like. 

198.  Sensation  and  Behavior. — All  the  Protozoa  show  more 
or  less  sensitiveness  to  external  conditions.  They  may  be 
caused  to  contract  and  move  by  mechanical  stimuli  such  as 
contact  or  jarring,  by  chemically  active  substances  in  the  water, 
by  light,  by  changes  in  temperature,  and  the  like.  Vorticella 
and  Spirostomum  are  exceedingly  sensitive  to  contacts;  Amoeba 
avoids  the  light;  many  forms  seem  to  find  their  food  as  the 
result  of  the  chemical  differences  in  the  water  and  may  be 
seen  to  swarm  about  suitable  objects;  the  contractile  vacuoles 
of  many  forms  contract  more  rapidly  in  warm  than  in  cold 
water;  Paramecia  tend  to  collect  in  groups  at  the  edge  of  the 
cover-glass,  around  air-bubbles,  about  green  filaments,  or 
even  without  any  foreign  matter  whatever.  So  far  as  we 
know,  these  simple  responses  do  not  give  evidence  of  special 
organs,  but  merely  represent  a  diffused  protoplasmic  irritability 
and  power  of  responding  to  stimuli  (§§20,  21). 

On  the  whole,  when  protozoa  are  stimulated,  their  response 
is  an  advantageous  one.  That  is  it  is  positive,  or  toward  sub- 
stances or  forces  that  are  of  help  to  it;  and  negative  to  stimuli 
that  are  hurtful.  It  is  not  believed  that  the  protozoan  is  con- 
scious of  these  conditions.  It  probably  means  that  they  have 


156 


ZOOLOGY 


inherited  the  tendencies  which  through  untold  generations  have 
resulted  in  safety.  Those  with  wrong  tendencies  have  been 
eliminated.  In  this  way,  through  generations  of  trial  and  error 
and  by  adjustments  on  the  part  of  the  organisms  they  have 
become  adapted  to  the  conditions  of  life.  We  have  no  evidence 

FIG.  69. 


FlG.  69.  Paramecium.  i,  transverse  fission;  2-5,  stages  in  conjugation.  Lettering  as  in  Pig. 
68.  The  meganucleus  gradually  disintegrates  during  the  process  and  the  micronucleus  by  two 
successive  divisions  forms  four  micronuclei.  Two  of  these  disintegrate.  One  of  the  remaining 
micronuclei  (n»)  in  each  animal  passes  into  the  other  Paramecium  and  unites  with  the  stationary 
micronucleus  (n*).  thus  fertilizing  it.  Later  a  new  meganucleus  is  formed  in  each  animal  by  the 
division  of  this  body.  Nucleus  n*  is  often  smaller  than  n«  and  may  represent  the  male  element. 

Questions  on  the  figure. — Which  is  permanently  represented  in  the  cell  during 
conjugation,  the  micro-  or  the  mega-nucleus?  Which  seems  to  correspond  most 
nearly  to  the  ordinary  nucleus  of  higher  forms?  What  really  transpires  in  the  act 
of  conjugating?  Compare  this  with  more  elaborate  figures  and  descriptions  in 
reference  texts. 

that  they  learn  by  experience.  .  They   transmit  their  native 
qualities,  but  not  the  results  of  accident  to  the  individual  body. 

199.  Reproduction. — In  the  Protozoa  we  discover  methods 
of  reproduction  which  are  to  be  looked  upon  as  suggestions  of 


PROTOZOA 


/ 
157 


FIG.  70. 


FIG.  71. 


FIG.  70.  A,  Vorticella,  a  stalked  ciliate  Infusorian:  i,  contracted;  2,  extended.  /,  food 
"vacuoles";  g,  gullet;  m,  contractile  fibre  (muscular);  w,  nucleus;  o,  mouth,  surrounded  by  ciliated 
disc;  p.v.,  pulsating  vacuole;  s,  stalk.  B,  a  colonial  type  similar  to  Vorticella. 

Questions  on  the  figures. — Compare  the  internal  structure  of  Vorticella  with 
that  of  Paramecium  (Fig.  68).  What  are  the  principal  differences?  Likenesses? 
How  is  a  colonial  type  (as  B}  formed?  How  are  new  colonies  started?  In  what 
way  does  the  animal  become  extended  after  contraction?  Compare  living 
animal. 

FIG.  71.  A,  Euglena  viridis,  a  flagellate  Infusorian.  i,  typical  swimming  condition;  2,  some- 
what contracted;  3,  spherical  resting  condition;  4,  encysted  stage  in  which  fission  has  taken  place. 
c,  cyst;/,  flagellum;  n,  nucleus;  o,  mouth;  p.v.,  pulsating  vacuole;  sp,  pigment  spot. 

B,  Podophrya,  a  stalked  Infusorian  bearing  tentacles  (<)•  P>  Infusorian  captured  for  food;  s, 
stalk. 

Questions  on  the  figures. — How  does  multiplication  in  Euglena  differ  from  that 
of  Paramecium?  What  are  the  differences  in  the  method  of  feeding  employed  in 
Vorticella  and  in  Podophrya?  What  is  the  structure  and  function  of  the  tentacles 
in  the  latter? 


158  ZOOLOGY 

methods  found  in  the  Metazoa.  Reproduction  among  the 
Protozoa  is,  primarily,  mere  fission  or  division  of  the  cell-sub- 
stance. In  some  instances  this  division  is  little  more  than  an 
irregular  breaking  up  or  fragmentation  of  the  protoplasm.  In 
others,  one  or  more  buds  may  arise  from  the  parent  cell.  A 
more  typical  method  is  by  the  equal  division  of  the  parent  into 
two  new  individuals.  In  still  other  instances,  especially  among 
the  Sporozoa,  there  is  the  formation  of  a  cyst,  within  which 
the  protoplasm  rearranges  itself  in  numerous  small  bits.  These 
finally  break  from  the  cyst  as  new  individuals.  In  all  such 
cases  the  old  nuclear  material  is  distributed  among  the  daughter 
individuals. 

There  are  some  indications  that  the  process  of  division 
carried  on  for  a  long  time  without  cessation  results  in  a  gradual 
loss  of  the  vitality  of  the  stock.  There  are  three  ways  in 
which  this  untoward  result  is  overcome,  so  that  a  kind  of  re- 
juvenation occurs.  In  the  first  place,  a  thick  wall  may  be 
formed  and  a  period  of  rest  ensue  (encystment).  It  appears 
also  that  new  vitality  may  be  given  to  a  culture  of  paramecia 
by  rhythmic  changes  in  the  conditions  of  life,  particularly  in 
the  foods.  Or  in  the  third  place,  there  may  be  a  temporary 
(Paramecium)  or  permanent  (Volvox,  Vorticella)  union  of  two  or 
more  individuals.  This  is  conjugation.  The  essential  thing  in 
conjugation  seems  to  be  the  introduction  of  new  nuclear  matter 
into  the  cell.  The  conjugation-cells  (gametes)  may  be  alike 
(Paramecium),  or  diverse  (Vorticella  or  Volvox). 

Paramecium  may  reproduce  for  many  generations  by  divi- 
sion, and  then  two  individuals  may  conjugate,  exchange  certain 
nuclear  elements,  and  separate,— beginning  once  more  their 
process  of  division.  There  is  here  no  sign  of  sexual  dimorphism 
in  the  Paramecia  themselves.  It  has  been  discovered,  however, 
that  the  portion  of  the  nucleus  which  passes  out  of  each  con- 
jugant  into  the  other  is  smaller  than  that  with  which  it  unites. 
It  might  therefore  be  considered  a  male  nucleus. 

In  the  colonial  species,  as  Vorticella  and  Volvox,  there  is 
the  union  and  permanent  fusion  of  the  whole  protoplasm  of 
individuals  (cells),  distinctly  different  in  form  and  size,  to 
produce  the  new  individual.  This  is  much  like  the  dimorphism 


PROTOZOA 


159 


found  in  the  sexual  cells  in  the  Metazoa  or  many-celled  animals, 
and  illustrates  heterogamy  (see  §101).     Consult  Figs.  7,  69,  73. 

200.  History. — The   existence   of   the    Protozoa   was   prac- 
tically unknown  until  the  compound  microscope  came  into  use. 

FIG.  72. 


FIG.  72,      Eudorina.     A  colony  of  16  flagellate  cells  imbedded  in  a  gelatinous  matrix. 


FIG.  73- 


FIG.  73.  Eudorina.  The  development  of  reproductive  bodies  within  the  colony  from  the 
ordinary  vegetative  cells  (»).  /,  a  mass  of  female  cells;  m,  a  mass  of  male  or  motile  cells;/',  a  single 
female  cell  surrounded  by  male  cells  (mO;  w,  the  boundary  of  the  original  colony. 

Questions  on  figures  72  and  73. — What  suggests  that  this  is  a  colony  rather 
than  an  individual?  What  suggests  the  reverse?  Compare  accounts  in  other 
texts  to  test  your  conclusions.  What  degree  of  differentiation  is  shown  among  the 
cells? 


l6o  ZOOLOGY 

A  naturalist  of  Holland,  Leeuwenhoek,  first  discovered  the 
Infusoria,  and  thus  opened  up  one  of  the  most  interesting 
departments  of  zoology.  It  was  not  until  the  middle  of  the 
nineteenth  century  that  the  simple,  unicellular  structure  of 
the  Protozoa  was  really  understood.  Many  of  them  can  endure 
drying,  be  blown  about  in  the  spore  stage,  and  then  take  up 
active  life  again  on  the  return  of  water,  so  that  thereupon,  in  a 
few  hours,  Infusoria  may  literally  swarm  where  none  seemed  to 


FIG.  74. — A  compound  Foraminiferan — Nodosaria.  a,  aperture  of  shell;  /,  food  particles 
captured  by  the  strands  of  protoplasm  outside  the  shell;  n,  nucleus;  sh,  shell.  1-4,  the  successive 
chambers  of  the  shell;  i,  being  the  oldest. 

Questions  on  the  figure. — Does  this  seem  a  colony  or  a  single  individual? 
Why  ?  Why  is  digestion  possible  outside  the  capsule  ?  Compare  this  with  figures 
of  Protozoa  in  which  there  is  no  large  aperture  to  the  shell. 

be.  This  is  responsible  for  the  long  life  of  the  old  belief  that 
they  arose  by  "spontaneous  generation,"  that  is,  without 
parents.  It  is  only  in  recent  years  that  this  belief  has  been 
finally  disproved.  It  is  known  that  they  do  not  appear  in 
water  that  has  been  boiled  and  kept  free  from  exposure  to  the 
air.  Much  brilliant  work  has  been  done  on  the  group  in  recent 
years,  on  structure,  on  behavior,  and  on  their  relation  to  disease. 

201.  Classification  of  Protozoa. — The  following  are  the  principal  classes  of 
Protozoa. 


PROTOZOA 


161 


Class  I.  Rhizpoda  (root-footed). — Type:  Amoeba.  The  Rhizopoda  are  amoeboid 
in  form  with  pseudopodia,  which  may  be  either  blunt  (Fig.  67)  or  slender  (Fig.  74). 
The  protoplasm  may  be  naked  (Amceba)  or  may  secrete  a  shell  either  calcareous 
(Foraminifera)  or  siliceous  (Radioloria).  In  the  shelled  forms  the  pseudopodia 
pass  out  through  openings  in  the  skeleton  (Fig.  75).  Reproduction  is  usually  by 
division,  or  by  the  formation  of  many  spores.  Encystment  frequently  occurs. 

Class  II.  Mastigophora  (whip  bearers).  Types:  Euglena,  Chilomonas,  Volvox, 
Trypanosoma.  Active  protozoa  which  may  be  simple  or  colonial.  They  bear  one 
or  more  large  lashes  or  flagella.  The  trypanosomes  are  blood-parasites. 

Class  III.  Infusoria  (in  infusions). — Types:  Paramecium,  Stentor,  Vorticella. 
Predominantly  active  protozoa,  usually  without  shell,  but  with  distinct  cortical 
portion  from  which  project  permanent  vibratile  threads  of  protoplasm  (cilia, 
flagella,  or  tentacles),  from  the  possession  of  which  the  sub-classes  are  named. 

FIG.  75. 


PIG.  75.  Actinomma,  a  radiolarian  with  a  shell  and  no  mouth.  A,  whole  animal  with  a  portion 
of  two  spheres  of  shell  removed.  B,  section,  showing  relation  of  protoplasm  to  the  skeleton,  c., 
central  capsule;  n,  nucleus;  p,  protoplasm;  o,  openings  through  which  the  pseudopodia  extend. 
(From  Parker  and  Haswell.) 

There  is  usually  a  permanent  mouth.  The  nucleuses  always  present  and  assumes 
a  great  variety  of  shapes.  The  infusoria  are  typically  free-swimming,  but  many 
are  capable  of  attachment  by  a  contractile  stalk,  to  foreign  objects  (Vorticella). 
Reproduction  is  normally  by  equal  division,  but  budding  and  spore  formation 
occur.  Conjugation  is  common,  and  may  be  either  temporary  or  permanent. 

Class  IV.  Sporozoa  (spore- animals). — Types:  Plasmodium  vivax,  Gregarina. 
Protozoa  predominantly  passive  in  habit,  parasitic,  with  no  pseudopodia,  and  no 
cilia  in  the  adult.  Remarkable  for  encysted  resting  stages  and  spore  formation. 
Conjugation  often  precedes  the  formation  of  the  cyst. 

202.  Place  in  Nature. — Protozoa  are  an  important  element 
in  the  food  of  many  aquatic  animals.  Despite  their  minute 
size,  their  immense  numbers  and  universal  distribution  make 


l62  ZOOLOGY 

them  important.  Together  with  bacteria  they  serve  to  save 
for  the  organic  world  much  decaying  material  which  no  other 
animals  could  utilize.  The  bacteria  decompose  organic  matter 
and  the  protozoa  devour  bacteria.  They  in  turn  become  food 
for  higher  animals.  We  have  seen  that  there  are  green  forms 
that  manufacture  their  own  food.  Some  live  on  debris,  some 
are  predatory,  some  are  parasitic,  and  some  are  symbiotic  with 
algae.  Rhizopod  shells  dropping  to  the  bottom  of  the  ocean 
form  the  "ooze," — the  chalk  of  later  geological  epochs.  Other 
forms  of  limestone  also  are  produced  by  the  accumulations  of 
these  calcareous  shells.  Similar  masses  of  the  siliceous  shells 
occur  in  various  parts  of  the  earth. 

Some  of  the  Protozoa,  especially  the  parasitic  Sporozoa 
produce  diseases  in  man  and  other  animals.  Malaria  and 
possibly  yellow  fever  in  man  are  caused  by  Sporozoa  in  the 
blood.  In  both  these  diseases,  species  of  mosquitoes  are 
apparently  the  cause  of  the  introduction  of  the  spores  into  the 
human  system.  Texas  fever,  one  of  the  most  dreaded  of  the 
diseases  of  cattle,  is  believed  to  be  communicated  through  the 
cattle  tick,  in  which  the  sporozoan  producing  the  disease 
undergoes  a  portion  of  its  life  history.  Trypanosomes,  flagel- 
late blood-parasites,  are  responsible  for  "sleeping  sickness". 
in  man  in  tropical  regions.  Similar  parasites  are  found  in  the 
blood  of  rats  and  other  animals. 

Amoeba-like  rhizopods  in  the  intestine  of  man  cause  some 
forms  of  dysentery  and  other  derangements  of  the  tract.  Similar 
organisms  accompany  small-pox,  hydrophobia,  pyrea,  and 
other  disease,  though  it  is  not  known  whether  they  have 
an  active  influence  on  the  diseases. 

Pieces  of  such  protozoa  as  Stentor  have  been  shown  to  be 
able  to  regenerate  a  whole  animal,  provided  a  portion  of  both 
nucleus  and  protoplasm  are  present,  but  not  otherwise.  This 
shows  that  each  is  necessary  to  the  activities  of  the  animal. 
Because  they  are  lowly  and  simple  animals,  we  must  not  con- 
sider that  they  are  either  unimportant  or  unsuccessful  in  the 
struggle  for  existence.  Their  wonderful  reproductive  power 
insures  that  they  hold  their  own  whenever  the  conditions  are 
at  all  favorable  for  them.  They  occur  in  practically  all  the 


PROTOZOA  163 

waters   of   the  earth,   increasing  or  decreasing   as   their  food 
varies  in  abundance. 

203.  Supplementary  Studies  for  the  Library. 

1.  The  reactions  of  Protozoa  to  light;  to  chemical  substances;  to  heat;  etc. 

2.  Their  power  of  resistance  to  heat;  cold;  drouth.     The  practical  results 
thereof. 

3.  The  economic  importance  of  Protozoa. 

4.  What  is  "plankton"?     What  is  the  importance  of  its  study? 

5.  Conjugation  in  Protozoa.     Compare  methods  of  reproduction  and  conjuga- 
tion in  the  various  groups.     Follow  the  nuclear  changes  in  conjugation  of  Para- 
mecium. 

6.  Why  should  Volvox  and  Euglena  be  considered  animals  rather  than  plants? 

7.  Diseases  in  man  or  animals  believed  to  be  caused  by  the  Sporozoa.    The  r61e 
of  the  mosquito  in  the  life  history  of  the  sporozoa  causing  malaria  and  yellow  fever. 
The  life  cycle  of  this  parasite.     The  bearing  of  these  facts  upon  infection  and  the 
management  of  these  diseases. 

8    Forms  of  the  Protozoa  of  different  classes  as  shown  by  the  illustrations  in  the 
larger  text-books. 

9.  The  varying  form  of  the  nucleus  in  different  species  of  Protozoa. 


CHAPTER  XI 

PHYLUM  II.— PORIFERA  (Pore-Bearing) 

LABORATORY  EXERCISES 

204.  Grantia. — This  is  a  marine  sponge  and  in  consequence 
the  majority  of  schools  will  be  compelled  to  depend  upon  alco- 
holic material.  Grantia  occurs  along  our  New  England  coast, 
and  is  found  attached  to  piles  or  to  stones  a  few  feet  below  the 
low- tide  mark.  If  the  school  is  near  the  coast  the  living  sponge 
should  be  studied  in  a  basin  of  sea-water. 

1.  General  Form. — (Keep   in   a  watch-glass,    covered   with 
the  preserving  fluid.)     Make  careful  outline  sketches  of  every- 
thing discovered. 

Note, — the  basal  or  attached  portion;  the  column;  the  free 
end.  How  do  the  ends  differ  ?  Are  there  any  openings  ? 

Do  you  find  any  connection  between  individuals  (budding)  ? 
Are  these  individuals  of  equal  size  ? 

2.  Structure. — Split  the   body  longitudinally  with   a  sharp 
scalpel,  and  examine  with  hand  lens  or  a  low  power  of  the 
microscope. 

Study, — body  wall;  cloaca  (internal  cavity);  the  relation 
of  the  cloaca  to  the  osculum  (the  opening  at  the  unattached 
end). 

By  what  is  the  osculum  surrounded  ?  Notice  in  the  wall  of 
the  cloaca  the  minute  openings  of  the  radiating  chambers.  Do 
they  communicate  with  the  exterior?  What  are  the  functions 
of  the  osculum  and  of  the  pores?  Evidences? 

3.  Make  thin  cross  sections  with  a  razor,  mount  under  cover-glass,  and  examine 
further  for  points  in  2.     Do  both  internal  and  external  pores  open  into  the  radial 
chambers?     Notice  the  spicules.     Is  there  any  regularity  in  their  arrangement? 
What  differences  in  shape  and  size  have  you  discovered  in  the  spicules  from  different 
regions  of  the  body? 

4.  Place  a  bit  of  the  sponge  in  a  small  amount  of  a  5  %  solution  of  caustic  potash 
and  boil      Examine  under  high  power,  and  draw  the  differently  shaped  spicules. 

5.  Place  a  bit  of  the  sponge  on  slide  and  allow  weak  acetic  or  hydrochloric  acid 
to  pass  under  the  cover.     Note  and  interpret  results. 

164 


PORIFERA  165 

205.  Comparison  Demonstrations. 

1.  Fresh-water  Sponge. — In  portions  of  the  country  where  the  streams  are  clear, 
swift,  and  with  rocky  bottoms,  a  fresh- water  sponge  may  often  be  found  which  will 
be  valuable  to  compare  with  Grantia  or  to  substitute  for  it.     It  grows  attached  to 
submerged  objects  and  is  commonly  of  a  dirty  greenish  color,  though  this  may 
vary.     This  sponge  is  firm  and  gritty  to  the  touch,  and  may  be  either  compact  or 
branched.     Use  the  general  outline  prepared  for  Grantia,  noting  the  points  of 
contrast.     Is  there  anything  like  the  osculum?     the  cloaca?     Gemmules  or  re- 
productive bodies  may  occur  imbedded  in  the  flesh,  especially  at  the  base. 

2.  The  Sponge  of  Commerce. — This  is  merely  the  skeleton  of  a  sponge  from  which 
all  the  cellular  part  has  been  removed.     Select  a  small  rounded  specimen.     Do  you 
find  any  signs  of  the  attached  end?  of  an  osculum?     Split  the  sponge  with  scissors, 
beginning  with  an  osculum.     Are  there  any  canals  as  in  Grantia?     If  so,  what  is 
their  arrangement?     Examine  a  small  portion  of  the  skeleton  under  the  micro- 
scope.    Test  as  before  (for  calcic  carbonate)  with  dilute  acid.     Is  the  skeleton  elas- 
tic?    Why? 

DESCRIPTIVE  TEXT 

206.  The    Protozoa   are   unicellular   animals,    or   at   most, 
masses  of  similar  cells  in  a  more  or  less  globular  form.     This 
condition  is  comparable  to  the  morula  stage  of  the  embryos  of 
higher  animals   (see   §54).     In  all  the  other  groups  (Metazoa) 
the  cells  at  some  stage  in  development  are  in  at  least  two 
layers,  an  inner  and  an  outer  or  superficial  layer,  a  structural 
condition  which  we  have  seen  at  its  simplest  in  the  gastrula 
(see  §55).     The  exact  position  of  the  Porifera  in  the  animal 
series  has  long  been  a  matter  of  debate,  but  the  great  majority 
of  zoologists  agree  that  they  stand  below  all  the  other  Meta- 
zoa,   presenting    transitional    features    between    the    Protozoa 
and  Metazoa.     For  this  reason  they  are  especially  interesting. 
Some  authors  include  them  with  the  next  phylum — the  Ccelen- 
terata.     They  possess  two  cell-layers,  but  the  division  of  labor 
among  the  cells  is  not  so  decided  as  in  the  Ccelenterata,  and 
the  individual  cells  are  very  much  more  independent  of  each 
other  in  consequence. 

207.  General  Characters. 

i.  Porifera  possess  a  system  of  internal  chambers  through 
which  the  water  flows.  The  water  enters  by  means  of  many 
minute  pores  at  the  surface,  passes  along  radiating  tubes  (in- 
current  channels)  to  the  central  cavity  (cloaca)  and  escapes 
through  one  or  more  larger  openings  (oscula)  at  the  unat- 
tached end.  There  is  no  true  ccelom  (see  §58). 


i66 


ZOOLOGY 


2.  Parts  are  arranged  about  the  central  cavity  but  not  usu- 
ally in  a  symmetrical  fashion. 

3.  There  are  two  distinct  layers,  ectoderm  and  entoderm. 
These  are  separated  by  a  gelatinous  middle  region  in  which  are 
included   cells  of  different  kinds   (mesenchyma  or  mesoderm) 


FIG.  76. 


FIG.  77- 


FIG.  76.     Leucandra,  a  simple  type  of  sponge.     (From  Delage  and  Herouard;  "Traitede  Zoologie 

Concrete.") 

Questions  on  the  figure. — What  is  the  position  of  the  osculum  ?  Which  is  the 
attached  end?  How  many  individuals  are  represented  in  the  cut? 

FIG.  77.  Diagrams  to  illustrate  the  development  of  one  of  the  simpler  types  of  sponge:  i,  the 
egg;  2,  section  of  16-  to  32-celled  stage;  3,  section  of  later  stage,  a  ciliated  larva  (blastula) ;  4,  gastrula; 
5.  section  through  older  larva  which  has  become  attached  by  the  end  containing  the  blastopore. 
New  openings  break  through  by  the  coalescence  and  perforation  of  the  ectoderm  and  entoderm, 
and  a  form  results  such  as  is  figured  in  Fig.  78.  a,  archenteron;  &/.,  blastopore;  ec.,  ectoderm; «»., 
entoderm;  mes.,  mesenchyma;  s,  segmentation  cavity. 

Questions  on  the  figures. — What  terms  would  be  applied  to  the  cleavage  and 
gastrulation  in  this  sponge?  What  is  suggested  as  to  the  mode  of  forming  meso- 
derm ?  The  attachment  of  the  sponges  by  the  blastopore  end  of  the  larva  necessi- 
tates what  later  development  ?  See  Fig.  78.  Examine  figures  in  other  texts  of  the 
development  in  other  species. 

not  in  a  true  layer.     In  the  cells  of  the  mesenchyma  spicules 
are  produced,  forming  the  supporting  skeleton  (Fig.  79,  C). 

4.  Non-sexual  reproduction  is  prevalent,  but  dimorphic 
sexual  cells  are  also  formed  in  the  mesenchyma.  The  sexually 
produced  larva  is  free-swimming;  the  adult  is  attached. 


PORIFERA 


I67 


5.  Mostly  marine;  wholly  aquatic. 

208.  General  Form. — The  simpler  sponges  are  cylindrical 
or  vase-shaped  sacs  with  an  opening  (the  osculum)  at  the  un- 
attached end.  From  the  central  cavity  (cloaca)  of  the  sac 
numerous  radial  passages  pierce  the  walls  (Fig.  78),  and 
terminate  directly  or  indirectly  in  pores  at  the  surface  (whence 


_E.Jb. 


FIG.  78.  Diagram  of  simple  type  of  sponge,  more  mature  than  in  Fig.  77.  c,  cloaca;  ch, 
chambers,  lined  with  flagellate  entoderm;  e.p.,  external  pores;  i.p.,  internal  pores;  mcs.,  mesenchyma; 
o,  osculum;  r.c.,  radiating  canals.  Other  letters  as  in  Fig.  77.  In  .the  adult  sponge  the  canals  and 
flagellate  chambers  become  much  more  complex  than  figured  here. 

Questions  on  the  figure. — What  portions  of  the  animal  are  lined  with  ectoderm? 
With  entoderm?  What  two  main  types  of  entoderm  are  figured?  What  is  the 
actual  nature  of  the  mesoderm  in  sponges?  Is  there  a  coelom  (a  cavity  bounded  by 
mesoderm)  ?  What  mechanical  advantage  do  you  see  in  the  fact  that  the  water 
currents  enter  by  way  of  the  radial  canals  and  find  their  exit  through  the  osculum, 
rather  than  the  opposite  direction?  Compare  with  Pig.  79. 

the  name — Porifera).  In  the  more  complicated  sponges  there 
is  such  power  of  budding  and  lateral  growth  that  there  is 
formed  a  dense  tuft  of  sponge  made  up  of  many  individuals 
in  organic  connection  with  each  other.  In  such  sponges  the 
simplicity  of  the  internal  structure  is  lost,  and  the  cloaca  may 
branch,  opening  to  the  exterior  by  a  number  of  oscula.  The 
radial  passages  which  penetrate  the  wall  become  much  branched 


l68  ZOOLOGY 

and  enlarged  in  special  regions  until  the  mesenchyma  becomes 
honey-combed  with  the  passages  and  chambers.  No  animals 
are  more  profoundly  influenced  by  their  environment,  in 
respect  to  the  special  form  which  the  individual  or  colony 
assumes,  than  the  sponges.  Individuals  which  develop  in 
active  currents  differ  much  in  bodily  shape  from  members 
of  the  same  species  which  grow  in  sheltered  places.  In  all 
instances  the  form  assumed  appears  to  be  correlated  to  the 
external  conditions. 

209.  The  Structure  of  the  Body. — In  the  typical  condition 
the  sponge  consists  of  an  outer  epithelium  and  its  derivatives 
(ectoderm),  an  inner  epithelium  (entoderm),  and  an  unorganized 
middle  region  (mesenchyma).  From  certain  unusual  occur- 
rences in  the  early  development  of  sponges  we  are  not  sure 
that  the  ectoderm  and  entoderm  in  them  are  homologous  with 
those  layers  in  the  animal  kingdom  generally. 

The  outer  epithelium  is  usually  of  flattened  cells.  These 
cover  the  whole  outer  surface  and  line  the  incurrent  canals. 
From  this  layer  arise  certain  specialized  cells  which  come  to 
lie  deeper  and  even  to  invade  the  mesenchyma.  Among  these 
are  fibrous  contractile  cells,  and  cells  that  secrete  the  hard  parts, 
as  spicules  of  lime  and  spongin  fibres.  In  the  middle  region 
are  also  amoeboid  cells, — which  ingest,  store,  and  convey  foods, — 
and  reproductive  cells.  The  inner  epithelium,  or  entoderm, 
lines  the  general  cavity  and  the  tubes  and  chambers  which 
penetrate  the  body  wall.  In  the  cloaca  the  entoderm  is  flat- 
tened; but  in  the  radial  canals  it  is  columnar  or  flask-shaped, 
collared,  and  flagellate  (Fig.  79,!)).  These  cells  by  means  of 
their  flagella  create  the  inward  currents  of  water  that  bring 
food  to  the  animal. 

In  the  mature  specimen  all  these  tissues  are  penetrated 
and  supported  by  the  spicules  or  threads  of  secreted  skeleton. 
These  may  be  calcareous,  siliceous,  or  horny.  The  sponge  of 
commerce  illustrates  the  last.  The  spicules  may  be  isolated 
and  independent  as  in  Grantia,  or  become  fused  into  a  continu- 
ous framework.  But  for  this  framework  the  otherwise  soft 
animal  would  collapse  into  a  shapeless  mass  and  thus  close  the 
openings  whereby  water  brings  the  oxygen  and  food.  It  is  the 


PORIFERA  169 

form  of  the  skeleton  too  which  gives  the  characteristic  form  to 
the  individuals  and  colonies  of  the  different  species. 

210.  Nutrition. — The  food  of  sponges  is  essentially  similar 
to  that  of  the  single-celled  Protozoa.     It  is  carried  in  by  the 
water  currents,  which  enter  the  pores,  pass  along  the  canals 
lined  with  the  collared  flagellate  entoderm  into  the  cloaca,  and 
from  there  reach  the  exterior  by  way  of  the  osculum.     The  food 
particles  are  taken  up  principally  by  the  entoderm  cells  lining 
the  radial  chambers  and  by  the  amoeboid  cells  which  belong  to 
the  mesenchyma.     In  these  cells  digestion  takes  place  as  in 
Amoeba.     The  indigestible  parts  of  the  food  are  returned  to 
the  current  and  are  eliminated  through  the  osculum.     There  is 
no  other  circulation.     The  digested  food  apparently  diffuses 
from  cell  to  cell  or  is  carried  by  the  amoeboid  cells.     Respira- 
tion occurs  through  all  the  cells  which  are  in   contact  with 
the  water. 

211.  Sensation  and  Motion. — Sponges  are  fixed  and  vege- 
tative in  their  adult  life,   and  show  very  little  of  the  more 
active  functions.     In  addition  to  the  ciliated  and  amoeboid  cells 
already   described,   the  pores   may  be   closed  in  response   to 
stimulus.     Contractile  elements  have  been  described  as  occur- 
ring in  the  epithelium  of  these  regions.     It  is  uncertain  whether 
there  are  any  nervous  elements.     The  oscula  open  and  close 
under  certain  conditions,  the  flagellate  cells  work  in  unison,  and 
their  rate  may  be  caused  to  vary  by  change  of  conditions. 

212.  Reproduction  by  outgrowth  or  budding  is  common. 
In  this  way  large  colonies  arise  from  a  single  individual.     New 
colonies  may  arise,  especially  in  the  fresh-water  sponges,  by 
the  separation  of  gemmules  or  groups  of  cells  produced  asexu- 
ally  within   the   mesenchyma.     These,  after  a  period  of  rest, 
escape  and  produce  new  individuals.     Sexual  reproduction  also 
occurs  in  all  sponges.     The  ova  and  sperm  are  developed  in 
the  mesenchymatous  layer.     The  male  and  female  cells  origi- 
nate from   the    same    individual    (hermaphroditism) .     Usually 
however  the  sexes  mature  at  different  times. 


170 


ZOOLOGY 


213.  Development. — Fertilization  of  the  ovum  and  early 
cleavage  take  place  in   the   mesenchyma  near  the  incurrent 


FIG.  80. 


FIG.  79.  Diagrams  showing  the  arrangement  of  the  radiating  canals  in  two  types  of  sponges: 
A,  Ascon  type;  B,  Sycon  type;  C,  a  portion  (*)  of  the  latter,  more  highly  magnified,  showing  char- 
acter of  the  three  layers,  ec.,  ectoderm;  en1  entoderm  (flattened  layer);  en2,  flagellate  entoderm; 
e.p.,  external  pores;  f.c.,  flagellate  chambers  of  the  radiating  canals;  i.p.,  internal  pores;  mes., 
mesenchyma;  r.c.,  radiating  canals;  D,  two  flagellate  cells  more  highly  magnified.  After  Korschelt 
and  Heider. 

Questions  on  the  figures. — Trace  the  relation  of  ectoderm  to  the  entoderm  in 
these  two  types?  Compare  these  with  illustrations  in  reference  texts.  Is  there 
any  way  of  accounting  for  this  disproportionate  growth  of  the  entoderm  ?  What 
are  the  apparent  functions  of  the  flagellate,  collared  epithelium  ?  What  structures 
are  to  be  found  in  the  mesenchyma  in  sponges? 

FlG.   80.      Axinella  polypoides,  showing  numerous  oscula.      After  Schmidt. 

Questions  on  the  figure. — What  are  the  principal  external  differences  between 
Axinella  and  Leucandra  (Fig.  76)?  How  many  individuals  are  represented  here? 
What  are  the  grounds  for  your  answer?  Compare  this  with  the  skeleton  of  the 
sponge  of  commerce. 


canals,   by   means   of   which   the   spermatozoa  find   entrance. 
Cleavage  is  total  and  for  the  most  part  equal   (see   §53),  pro- 


PORIFERA  171 

ducing  an  oval  blastula  which  swims  freely  by  means  of  cilia 
or  flagella.  While  there  are  some  peculiar  features  about  the 
gastrulation,  a  gastrula  or  two-layered  embryo  is  ultimately 
formed.  Strangely  enough  it  is  the  flagellate  cells  of  the 
larva  that  become  the  inner  or  entodermal  layer  of  the  adult. 
Conversely,  the  cells  which  in  other  Metazoa  give  rise  to  the 
entoderm  form  the  outer  epithelium  of  sponges. 

The  embryo  finally  settles  to  the  bottom  and  becomes 
attached  by  the  end  containing  the  blastopore,  which  thus 
becomes  obliterated  (Fig.  77,  bl).  An  excurrent  pore  breaks 
through  at  the  opposite  end,  and  the  numerous  incurrent 
pores  are  formed  at  the  sides.  The  mesenchyma  seems  to 
be  formed  by  special  blastomeres  or  by  cells  which  migrate 
from  the  other  layers  into  the  segmentation  cavity,  thus  filling 
it.  The  entoderm  outpockets  into  the  mesenchyma,  establish- 
ing connection  with  the  ingrowing  ectoderm,  thus  forming  the 
incurrent  canals  (see  Fig.  78).  In  most  species  the  process  is 
much  more  complex  than  that  described  here. 

214.  Classification. 

The  divisions  of  the  group  Porifera  are  made  on  the  basis  of  the  differences  in 
the  skeleton.     Two  principal  classes  may  be  recognized,  as  follows: 

I.  Calcarea. — Sponges  in  which  the  skeleton  is  composed  of  calcareous  spicules. 
Laboratory  type, — Grantia. 

II.  Non-Calcarea. — Sponges  with  glassy   (siliceous)   spicules,  or  with  homy 
(spongin)  fibres,  or  with  merely  a  gelatinous  mesenchyma.     Laboratory  types: — 
the  Venus  Flower-basket;  the  fresh- water  sponge;  the  commercial  sponge. 

215.  Ecology. — Sponges  are  chiefly  marine  animals,  and 
flourish  in  all  the  seas  and  at  any  depth.  The  larger  horny 
sponges,  of  which  the  bath  sponge  is  the  skeleton,  are  found  in 
the  warmer  seas  and  in  relatively  shallow  water.  The  best 
ones  are  found  in  the  Mediterranean,  in  the  South  Indian  Ocean, 
and  about  Florida  and  the  West  Indies.  By  reason  of  their 
budding  and  branching,  the  sponges  form  immense  colonies 
or  beds,  and  many  other  forms  of  life  associate  with  them  in 
varying  degrees  in  intimacy.  Worms,  Crustacea,  and  the 
larvae  of  many  forms  play  hide-and-seek  and  burrow  in  the 
thickets  of  growth  produced  by  sponges.  Several  species  of 
crabs  allow  their  shells  to  be  covered  by  sponges.  In  this  way 
they  are  less  conspicuous.  In  other  cases  the  sponges  may  pro- 


172  ZOOLOGY 

tect  the  crab  by  its  ill  odor  or  taste.  Sponges  have  no  power  of 
attacking  any  but  microscopic  organisms.  They  may,  however, 
smother  by  their  growth  fixed  animals  like  the  oyster.  Because 
of  their  harsh  spicules  or  their  unpleasant  secretions  they  are 
rarely  used  for  food  by  other  animals. 

The  horny  skeleton  of  certain  species,  the  "sponges"  of 
commerce,  is  the  chief  contribution  of  the  group  to  human  uses. 
Fossil  sponges,  apparently  of  the  same  general  characteristics 
as  those  now  living,  are  found  in  very  early  geological  strata. 

216.  Supplementary  Library  Studies. 

1.  Economic  value  of  sponges.     Sponge  fisheries.     Sponge  "farming."     The 
mode  of  preparing  sponges  for  market. 

2.  What  arguments  may  be  advanced  for  considering  the  sponges  as  colonial 
Protozoa?     What  is  the  conclusive  argument  for  regarding  them  as  Metazoa? 

3.  By  comparing  the  figures  of  sponges  found  in  your  reference  books,  note  the 
different  degrees  of  development  of  the  passages  lined  with  entoderm  and  ectoderm 
in  the  walls  of  various  species. 

4.  In  what  special  ways  do  sponges  become  adapted  to  the  conditions  in  which 
they  are  situated?     Effect  of  rapid  currents  on  them?     Of  quiet  water?     Of 
muddy  water? 


CHAPTER  XII 

PHYLUM  III.— CCELENTERATA  (HOLLOW  INSIDE).     (HYDROIDS,  CORALS, 

JELLY-FISHES,  ETC.) 

LABORATORY  EXERCISES 

217.  Hydra. — Hydras  are  small  tubular  animals  found  in 
permanent  fresh-water  pools,  attached  to  submerged  leaves, 
twigs,  algae,  etc.  They  are  somewhat  difficult  to  recognize 
when  disturbed  because  they  contract  into  small  rounded 
masses,  close  against  the  supporting  object.  Promising  ma- 
terials should  be  collected  from  several  ponds,  and  placed  in 
shallow  vessels  (a  white-ware  dish  is  good),  and  in  a  short 
time  the  hydras  will  become  extended.  The  green  hydra 
(H.  viridis)  is  perhaps  more  common  and  hardier,  but  is  not 
so  satisfactory  for  general  laboratory  work  as  the  brown  (H. 
fusca),  because  it  is  less  transparent. 

i.  Study  the  living  animal  in  a  glass  jar  (tumbler). 

Is  it  free  or  attached?  What  happens  if  it  is  freed  from  its 
attachment?  Is  it  lighter  or  heavier  than  the  water?  Evi- 
dences. Can  it  move  from  one  portion  of  the  vessel  to  another  ? 
If  so,  does  it  become  detached  ?  Watch  same  individuals  from 
day  to  day.  What  is  its  position  in  the  water?  If  the  vessel 
containing  hydras  be  placed  near  the  window  for  several  days, 
at  which  side  of  the  vessel  do  the  animals  become  collected? 
When  the  animals  are  stretched  out  at  their  greatest  length, 
touch  lightly  the  tip  of  one  of  the  tentacles.  Touch  the  body. 
Repeat  the  experiment  until  you  are  sure  of  your  results.  Note 
and  explain  as  well  as  you  can  the  results.  Of  what  degree  of 
contraction  is  the  animal  capable  ?  Do  you  notice  any  contrac- 
tions or  motions  of  parts,  when  the  hydra  is  undisturbed? 
What  seems  to  be  the  purpose  of  the  motions?  Evidences? 
Bring  a  piece  of  meat  the  size  of  a  pin-head,  or  a  Daphnia  or 
Cyclops,  in  contact  with  the  tip  of  a  tentacle  and  note  the  results. 
How  do  the  other  tentacles  behave  ?  Place  a  food-particle  di- 

i73 


174  ZOOLOGY 

rectly  at  the  base  of  the  tentacles.  How  is  it  swallowed  ?  How 
long  does  it  take?  What  becomes  of  it?  How  long  does  it 
remain  in  the  body  ?  Classify  the  results  which  you  have  at- 
tained, under  the  following  heads: — motion  and  locomotion, 
nutrition,  sensation.  Devise  still  other  experiments  to  test 
special  points  which  you  desire  to  know.  Have  you  dis- 
covered anything  that  would  lead  you  to  think  that  they 
learn  from  experience? 

2.  General  Structure. — Transfer  a  living  animal  to  the  slide, 
covering  it  with  a  drop  or  two  of  water.  Observe  with  a  low 
power  without  cover-glass.  Draw  carefully  in  outline  everything 
discovered. 

Note  body  regions. 

Foot  (attached  end). 

Column. 

Tentacles,  position,  number  (examine  several  specimens). 

Hypostome,  surrounded  by  the  tentacles. 

Mouth. 

To  what  extent  do  these  regions  vary  in  their  dimensions 
during  the  different  stages  of  contraction  of  the  hydra  ?  Would 
you  say  there  is  any  distinct  symmetry?  Which  is  the  main 
axis  ?  Is  there  any  indication  of  an  internal  (Castro-vascular) 
cavity  ?  What  is  its  extent  ?  Are  the  tentacles  solid  structures  ? 
Evidences  ?  Are  there  any  buds  in  your  specimen  ?  Relation  to 
the  parent  ?  To  what  extent  do  different  parts  of  the  body  do 
different  work  ? 

3.  Microscopic  Structure. — Cover  with  a  cover-glass  supported  by  objects 
nearly  as  thick  as  the  animal.  Study  with  a  higher  power.  Verify  the  points 
studied  above.  Follow  the  gastro-vascular  cavity  more  fully.  Is  there  an  aboral 
opening? 

Body  wall. 

Ectoderm,  or  outer  layer  of  cells. 
Entoderm,  or  inner  layer  of  cells. 

Determine  the  extent  of  each  layer.  Are  they  continued  into  the  tentacles? 
What  differences  do  you  find  in  the  thickness  of  the  layers  and  in  the  shape  and 
character  of  the  cells  of  each  layer  in  the  various  parts  of  the  body?  Is  there 
anything  between  the  ectoderm  and  entoderm? 

In  the  ectoderm,  especially  in  the  knobs  on  the  tentacles,  find  highly  refractive 
oval  bodies,  the  nettle  capsules.  Irrigate  with  a  drop  of  dilute  acetic  acid,  and 
watch  the  tentacle  all  the  while.  What  changes  have  occurred  in  the  nettle  cells? 


CCELENTERATA  175 

[A  whole  animal  stained  and  mounted  may  be  studied  profitably  in  comparison 
with  the  preceding.] 

4.  Histology  from  Sections. — If  the  teacher  is  not  equipped  for  imbedding  and 
sectioning  objects,  and  desires  to  carry  this  work  further,  stained  and  mounted 
sections  of  Hydras  and  most  of  the  other  prepared  sections  suggested  in  this  book 
can  be  secured  for  a  reasonable  sum  by  applying  to  any  of  the  large  laboratories. 
By  comparison  of  longitudinal  and  transverse  sections  verify  your  observations 
concerning  the  extent  of  ectoderm  and    entoderm.     What  occurs  between  the 
layers?     Study  the  shape  and  arrangement  of  the  cells  in  both  layers.     Compare  as 
to  size.     What  is  the  relation  of  the  nettle  cells  to  the  other  ectodermal  cells? 

5.  Histology  from   Maceration  Preparations. — Place  a  specimen  in  a  watch 
glass,  and  draw  away  some  of  the  water  with  a  pipette.     When  the  Hydra  is  well 
extended,  pour  over  it  an  aqueous  solution  of  hot  corrosive  sublimate.     Rinse 
and  place  in  Muller's  fluid  or  15%  alcohol  for  24  hours.     Take  a  portion  of  the 
body  and  place  on  a  slide  in  a  drop  of  glycerine  and  water.     Cover,  and  tap  the 
cover-glass  very  gently  with  a  needle.     The  cells  thus  become  separated,  and 
their  shape  may  more  readily  be  seen.     Instructions  for  staining  may  be  found  in 
texts  on  histology. 

Study  the  nettle  cells,  the  ectodermal  cells,  the  entoderm,  and  the  gland  cells 
of  the  foot  and  gullet. 

218.  For  comparison  with  Hydra  the  teacher  should  secure  some  alcoholic 
material  of  some  of  the  marine  hydroids,  as  Pennaria,  Obelia  or  Campanularia.     A 
few  slides  should  be  secured  bearing  whole  mounts  and  sections  properly  stained. 

The  following  points  should  be  studied  briefly:  Relation  between  individuals 
in  the  colonies, — branching.  What  classes  of  individuals  are  discoverable,  i.  e., 
how  do  the  different  branches  end?  Is  there  any  covering  to  the  softer  portions? 
Tentacles;  are  they  present?  If  so,  what  is  their  arrangement?  Hypostome? 
Mouth?  Is  there  a  gastro-vascular  cavity?  Ectoderm?  Entoderm?  Call 
attention  to  polymorphism  among  the  polyps  or  zooids. 

219.  Metridium  (Sea-anemone). — If  lack  of  appropriations  will  not  allow  the 
purchase  of  sufficient  material  for  class  work,  the  teacher  should  have  at  least  a 
few  well-hardened  and  preserved  specimens  of  sea-anemone.     From  these  should 
be  made  a  series  of  cross  sections  from  various  parts  of  the  body,  with  a  thickness 
of  one-eighth  to  one-fourth  inch.     These  sections  may  be  fastened  to  cards  or  to 
plates  of  mica  by  thread  or  fine  wire  and  kept  in  preserving  fluid.     One  specimen 
should  be  split  lengthwise,  and  one  left  whole.     Four  or  five  specimens  could  thus 
be  used  from  year  to  year  until  more  abundant  supplies  are  obtained. 

The  following  studies  should  be  made.  Make  drawings  to  illustrate  all  points 
made  out. 

1.  General  Form. 

Base,  or  aboral  disc  (the  end  attached  during  life). 
Column. 

Oral  disc:  zone  of  tentacles;  intermediate  zone;  lip-zone;  mouth;  siphono- 
glyphs  (grooves  in  the  angles  of  the  mouth), — number? 

2.  Transverse  Sections. 
Body  wall. 

Esophagus;  does  it  appear  in  all  the  sections?     Siphonoglyphs? 
Mesenteries.     How  is  the  esophagus  held  in  position?     What  differences 


176  ZOOLOGY 

do  you  find  in  the  mesenteries?  They  are  described  as  complete  (or 
primary),  and  incomplete  (or  secondary,  tertiary,  etc.). 
Show  by  a  diagram  the  number  and  arrangement  of  them,  especially  of  the 
primary.  Are  they  in  pairs?  Notice  the  inter-mesenteric  chambers. 
Can  you  find  the  muscular  thickenings  in  the  cut  mesenteries?  Sketch 
their  position.  Compare  with  conditions  figured  in  various  text-books. 

3.  Longitudinal  Section. 

Complete  your  study  of  the  structures  mentioned  above. 

Compare  the  complete  and  incomplete  mesenteries. 

Identify: 

Mesenteric  filaments  (on  free  edge  of  mesenteries). 

Genital  glands  (developed  in  the  substance  of  the  mesentery  near  the  edge). 

Ostia,  or  ring  canal;  openings  through  the  mesenteries  by  means  of  which 

the  mesenterial  chambers  communicate  with  one  another. 
Are  the  tentacles  solid  or  hollow? 

4.  General  Considerations. 

Make  diagrams  in  longitudinal  and  transverse  view  to  show  the  distribution 
and  connection  of  the  cavities  of  the  body.  Is  the  mouth  the  only  opening  into 
the  cavity?  Describe  the  symmetry  of  the  anemone.  Is  it  radial  or  bilateral? 
Give  reasons  for  your  answer. 

220.  Oculina  (or  other  branching  coral). — Study  the  branches  and  note  the 
position  of  the  polyps.  Is  the  arrangement  orderly?  If  so,  describe. 

Note  with  a  hand  lens  the  arrangement  of  the  septa,  which  grow  between  the 
fleshy  mesenteries  of  the  coral.  Compare  their  arrangement  with  that  of  the 
mesenteries  of  anemone. 

DESCRIPTIVE  TEXT 

221.  Some  authors  place  the  sponges  and  the  ccelenterates 
in  the  same  group  on  account  of  the  typical  barrel  shape,  the 
absence  of  a  true  ccelom  or  body  cavity,  the  somewhat  similar 
character  and  origin  of  the  middle  mass  (mesenchyma) ,  and 
the  agreement  of  the  principal  axis  of  the  adult  with  that  of  the 
gastrula.     In  the  ccelenterates  however  there  are  no  lateral 
pores.     The  principal  opening  serves  as  a  real  mouth  as  well 
as  vent  for  the  voiding  of  undigested  matter,  whereas  in  sponges 
it  is  not  a  mouth  in  any  sense.     In  general  the  individual,  even 
in  the  colonial  forms  of  ccelenterates,  is  more  distinctly  an  in- 
dividual than  in  the  sponges.     The  division  of  labor  among  the 
parts  and  the  interdependence  of  parts  is  greater  than  among  the 
sponges. 

222.  General  Characters. 

i.  A    single    system   of  internal   chambers    (gastro-vascular 


CCELENTERATA 


177 


cai'ity)   in   which   digestion   and   circulation  both   occur.     No 
ccelom. 

2.  Parts  radially  arranged  about  an  oral-aboral  axis.     Ten- 
tacles usually  occur  at  the  oral  pole  (Figs.  82,  85). 


FIG.  81. 


-   s 


f~ 


nu. 


PIG.  Si.  A,  T  iMj.il  •>!  in  ill  section  through  the  body  of  Hydra  (ffi^mniinlii)  B.  cmaB 
portion  of  the  wall  more  highly  magrrifird.  b,  bod;  ect^  ectodam;  cmL.  niUxIniu;  /.  foot;  fL. 
flagellcm;  g.i.,  gastro-  vascular  cavity;  »».,  moath;  met.,  mesenchyma  (noo-ceflnlarl;  m./-,  !••»  nlii 
processes  of  the  ectodennal  cells;  IB,  nettling  cells;  **,  gam**,  exploded;  mi  ,  mirl^»|«-  ff  ^»»*y^-  y 

Questions  on  the  figures.  —  How  many  cellular  layers  are  to  be  distinguished 
in  Hydra?  What  differentiations  are  represented  in  the  «**t**1flrm  in  different 

regions?  In  the  entoderm?  What  is  the  relation  of  the  bud  to  the  adult?  Why 
is  the  cavity  called  a  gastro-vascular  cavity?  How  is  contraction  effected  in 
Hydra? 

3.  A  supporting  layer  or  mass  (mesenchyma)  between 
ectoderm  and  entoderm,  sometimes  without  cells.  More  often 
cells  of  various  kinds  occur,  which  have  migrated  from  the 
other  layers. 

12 


I 78  ZOOLOGY 

4.  Nettle-cells   are   found  in  practically   the   whole  group 
(Fig.  83). 

5.  Nerve  cells  (sensory)  and  muscle  cells  both  occur. 

6.  Reproduction  by  non-sexual  methods  is  prevalent.     This 
often  alternates  regularly  with  the  sexual.     The  individuals  of 
the  two  generations  may  be  very  different  in  appearance  and 
habits. 

7.  Wholly  aquatic;  chiefly  mnriae. 

223.  General  Survey. — The  group  of  Ccelenterata  embraces 
animals  very  diverse  in  general  appearance,  which  may  never- 
theless be  reduced  to  two  types.  The  first  and  most  primitive 
is  the  tubular  hydroid  type.  This  is  sessile  and  is  essentially  a 
gastrula,  at  the  unattached  end  of  which  occurs  the  mouth, 
usually  surrounded  by  tentacles.  The  cavity  of  the  tentacles 
is  continuous  with  the  gastro- vascular  cavity  (Fig.  81).  Of  this 
type  we  may  distinguish  two  conditions:  (i)  in  which  the  in- 
dividuals (polyps)  occur  singly  (Hydra),  or  if  in  colonies,  the 
various  individuals  have  the  same  form  (as  the  corals);  (2) 
colonial  forms  in  which  the  individuals  making  up  the  colony 
are  very  different  (as  the  Siphonophora),  embracing .  open- 
mouthed  nutritive  individuals,  mouthless  reproductive  polyps, 
protective  polyps  abundantly  supplied  with  nettle-cells,  bladder- 
like  supporting  polyps,  etc.  (Figs.  86,  87).  The  extreme  con- 
ditions of  (i)  and  (2)  are  connected  by  forms  possessing  inter- 
mediate degrees  of  polymorphism.  Though  the  individual 
polyps  are  attached,  the  whole  colony  may  float  freely.  The 
second  type  is  the  active  jelly-fish,  or  medusoid  (bell)  type. 
The  medusae,  though  varying  greatly  as  to  details  agree  in 
having  a  shape  comparable  to  that  of  an  umbrella  or  a  bell 
(Fig.  82,  6).  The  convex  surface  is  normally  the  upper  surface. 
At  the  margin  of  the  umbrella  are  tentacles — often  very  numer- 
ous, and  frequently  much  elongated.  In  the  middle  of  the 
concave  surface  is  a  projection,  at  the  lower  end  of  which  is  the 
mouth-opening.  The  gullet  leads  from  the  mouth  into  a  cavity 
in  the  central  portion  of  the  body  of  the  bell  (gastro-vascular 
cavity).  From  the  central  cavity  radiating  passages  run 
through  the  substance  of  the  bell  to  the  margin  where  they 


CCELENTERATA 


179 


may  communicate  with  a  circular  canal  which  passes  around  the 
bell  near  the  bases  of  the  tentacles.  This  whole  internal  cavity 
is  lined  with  entoderm,  and  therefore  no  portion  of  it  represents 
a  ccelom,  but  is  merely  a  much-modified  digestive  tract  (Fig. 
82,  6). 

FIG.  82. 

-VH  CC '  T"  * — .-^ 


PlG.  82.  Sections  of  types  of  Ccelenterates  (diagrammatic):  T  (longitudinal)  and  2  (transverse) 
of  a  tubular  hydroid;  3,  Sea  Anemone  (longitudinal) ;  4,  same  (transverse,  at  the  level  of  the  upper 
dotted  line);  5,  same  (transverse,  at  the  level  of  the  lower  dotted  line);  6,  longitudinal  or  vertica 
section  of  a  Medusa;  7,  transverse  section  of  same  at  the  level  of  the  dotted  line.  The  continuous 
line  is  ectoderm,  the  broken  line,  entoderm,  and  the  stippled  portion,  mesenchyma.  c.c.,  circular 
canal;  g,  gullet;  g.p.,  gastro- vascular  cavity;  m,  mouth;  ma.,  manubrium;  mes.,  mesentery;  mes.1 
directive  mesentery;  o,  ostium;  r.  c.,  radial  canal;  t,  tentacle;  v,  velum. 

Questions  on  the  figures. — By  a  careful  comparison  of  the  diagrams  what 
points  of  similarity  do  you  find  in  these  three  types  ?  What  are  the  principal  points 
of  difference?  Examine  similar  diagrams  in  other  texts.  Why  is  Ccdenterate  an 
appropriate  name  for  all. 

The  bell  is  comparable  to  an  inverted  polyp  in  which  the 
main  axis  has  become  much  shortened,  accompanied  by  a 
thickening  of  the  body  in  the  direction  of  the  other  axes.1 
The  gastro-vascular  cavity  is  further  modified  by  the  increase 
of  the  mesenchyma  of  the  aboral  disc  and  by  a  union  of  the 
oral  and  aboral  walls  of  the  cavity  in  certain  regions.  The 

1  See  Textbook  of  Zoology,  Parker  and  Haswell,  Vol.  I,  p.  127,  Pig.  89. 


l8o  ZOOLOGY 

large  chambers  between  the  mesenteries  in  such  forms  as  the 
sea-anemone  thus  become  limited  to  small  radial  canals. 

Frequently  both  the  tubular  and  the  bell  types  are  found  in 
the  life  history  of  the  individuals  of  a  single  species.  The 
tubular  colonial  polyp  produces,  by  asexual  processes  such  as 
budding  or  fission,  the  bell  or  medusoid  forms  which  are  sexual. 
These  may  remain  attached  or  become  free  swimming.  They 
produce  ova  or  spermatozoa,  or  both,  and  from  the  sexual 
union  of  these  elements  the  non-sexual  tubular  polyp  is  again 
produced.  This  regular  alternation  of  sexual  and  sexless  in- 
dividuals is  known  as  alternation  oj  generation.  In  some  forms, 
however,  the  polyp  has  no  corresponding  bell  (as  in  hydra; 
corals;  sea-anemone),  and  for  some  bells  (as  in  some  large  pelagic 
medusae)  there  is  no  corresponding  polyp  stage. 

224.  The  nutritive  processes  in  the  Ccelenterata  are  marked 
by  relative  simplicity.  Food,  consisting  mainly  of  small  organ- 
isms and  organic  debris,  is  taken  into  the  mouth  often  with 
the  assistance  of  tentacles.  The  tentacles  are  frequently 
armed  with  numerous  special  cells  in  which  are  developed 
capsules  containing  long  stinging  threads,  with  poisonous  tips. 
When  these  threads  are  discharged  they  may  penetrate  and 
paralyze  small  organisms.  They  are  very  irritating  even  to 
the  human  skin.  In  some  types  of  nettling  capsules  the  thread 
forms  a  cork-screw  coil  that  may  take  hold  of  the  hairs  or  other 
projections  of  the  prey.  They  serve  as  organs  both  of  defense 
and  food  capture  (Fig.  83). 

Digestion  and  circulation  both  take  place  in  a  general 
cavity  (gastro-vascular)  lined  ztith  entoderm.  In  other  words  the 
circulatory  function  in  this  group  is  not  differentiated  from 
the  digestive.  In  the  colonial  forms  the  gastro- vascular  cavity 
of  the  various  polyps  in  the  colony  may  be  directly  continuous 
(Fig.  87).  Thus  a  kind  of  cooperative  digestion  occurs.  In 
the  medusa,  the  corals,  and  forms  like  anemone,  the  cavity  is 
much  more  complicated  than  in  the  tubular  hydroids,  on  account 
of  the  mesenteries.  The  entoderm  seems  to  take  up  food  from 
the  gastro-vascular  cavity,  in  part  at  least,  by  means  of  the 
amoeboid  action  of  some  of  the  entodermic  cells  a?  well  as  by 


CCELENTERATA 


181 


absorption.  Pseudopodia  are  formed,  and  particles  are  directly 
taken  into  the  body  of  the  cell.  Special  gland  cells  also  occur  in 
the  entoderm,  by  the  secretions  of  which  the  food  undergoes 
changes  preparatory  to  absorption.  There  is  no  anal  opening. 
Undigested  remnants  are  eliminated  at  the  mouth.  Respira- 

FIG.  83. 


P$?tt---  c 


—  nu. 


FIG.  83.  Nettling  cells  of  Hydra  (after  Schmeil).  A,  unexploded;  B,  exploded,  b,  barbs;  ct 
the  nettling  cell  in  which  the  nettling  organ  is  developed;  en.,  the  cnidocil  or  "trigger;"  cp.,  the 
capsule  or  nettling  organ;  /,  the  nettling  filament  or  lasso;  n,  neck  of  the  capsule;  nu.,  nucleus  of 
the  cell. 


Questions  on  the  figure. — Compare  the  parts  of  the  nettling  organ  before  and 
after  explosion  and  note  the  difference  in  position.  How  would  the  barbs  in  the 
neck  of  the  capsule  behave  as  it  is  forced  inside  out  by  the  compression  of  the 
capsule?  Find  from  your  reference  literature  the  nature  of  the  fluid  secreted  on 
the  inside  of  the  lasso. 


l82  ZOOLOGY 

tion — the  exchange  of  carbon  dioxide  for  oxygen — takes  place 
by  means  of  the  individual  cells  of  the  body  layers,  though  it 
is  probable  that  it  takes  place  more  satisfactorily  in  the  thin- 
walled,  more  actively  moving  tentacles.  Excretion  is  likewise 
a  general  body  function. 

225.  Motion. — All  the  Ccelenterata  are  supplied  with  con- 
tractile fibres.     Many  of  these  are  modified  ectodermal  or  ento- 
dermal  cells  rather  than  true  mesoderm   (Fig.   81,  B).     The 
fibres  run  both  longitudinally  and  transversely.     In  the  more 
active    types    cross-striate   fibres    may    occur.     The    attached 
(polyp)  forms  have  well-developed  longitudinal  fibres  in  the 
body  wall  and  the  mesenteries,  which  enable  the  soft  parts  of 
the  animal  to  be  drawn  close  to  the  supporting  object.     In 
the  medusoid  types  locomotion  is  effected  by  rhythmic  contrac- 
tions of  the  bell  as  a  whole.     By  this  means  the  water  is  expelled 
from  the  cavity  of  the  bell,  and  the  reaction  forces  the  animal 
forward. 

226.  Support. — The   attached   colonial   forms    (corals,    sea- 
fans,  etc.)  usually  possess  a  skeleton  of  calcareous  or  horny 
matter  commonly  secreted  by  the  ectoderm.     Each  polyp  con- 
tributes a  portion  to  the  common  skeleton — the  corallum.     The 
corallum  differs  greatly  in  form  in  the  different  species.     The 
particular  form  depends  on  the  manner  and  rate  of  budding  or 
non-sexual  reproduction  of  the  polyps,  and  the  activity  shown 
by  the  individual  in  secreting.     In  some  cases  single  polyps 
produce  a  skeleton   (cup-corals).     The  coral  reefs  of  tropical 
seas  are  illustrations  of  the  power  of  corals  to  form  and  excrete 
carbonate  of  lime.     Much  of  the  lime-stone  of  the  earth's  crust 
shows  that  corals  assisted  in  its  formation. 

227.  Sensation   and   Behavior. — The   nerve   cells   may   be 
scattered  diffusely  over  the  surface  of  the  body  with  a  mesh- 
work  of  fibrils  to  connect  them  with  the  muscular  and  nettle 
cells  and  with  each  other,  as  in  Hydra.     In  some  other  polyp- 
forms  there  is  more  differentiation  of  cells  and  fibres,  but  the 
elements  are  still  scattered.     In  the  more  active  types  there  is 
a  collection  of  the  cells  either  as  a  connected  ring,  or  in  groups, 


CCELENTERATA  183 

in  the  tentacle-bearing  rim  of  the  animal.  Associated  with  this 
collection  of  the  nervous  material  into  a  kind  of  nervous  centre, 
there  are  often  special  areas  of  sensory  epithelium,  or  sense 
organs,  developed  from  the  ectoderm.  It  is  not  wholly  clear 
what  kinds  of  stimuli  they  are  suited  to  receive  although  they 
are  designated  as  "eye  spots,"  or  as  "auditory"  or  "olfactory" 
pits.  Otocysts  (see  §m)  are  found  in  the  Ctenophores  and  in 
some  medusae,  and  apparently  function  chiefly  as  organs  of 
equilibration. 

Experiments  show  that  ccelenterates  are  sensitive  to  dif- 
ferences in  intensity  of  light,  to  mechanical  stimuli,  to  tempera- 
ture, to  gravity,  and  to  chemical  stimuli.  The  responses  may 
be  local,  as  when  a  Hydra  withdraws  its  tentacle  upon  a  light 
touch  or  uses  all  its  tentacles  and  the  region  about  the  mouth 
when  stimulated  mechanically  and  chemically  by  its  prey;  or 
general,  as  when  it  contracts  body  and  tentacles  completely 
upon  vigorous  mechanical  stimulation. 

In  addition  to  such  responses  to  external  stimuli  there  are 
in  Hydra  bendings  of  the  body,  wavings  of  the  tentacles,  and 
even  contraction  and  extensions  of  the  whole  body  that  suggest 
"seeking"  movements.  Hungry  specimens  are  more  active 
than  well-fed  ones.  All  of  this  shows  that  even  in  the  simplest 
representative  of  the  group  there  is  effective  nervous  and 
muscular  coordination. 

228.  Reproduction  and  Development. — The  occurrence  of 
both  sexual'  and  asexual  methods  of  reproduction  has  already 
been  mentioned  (§223).  It  is  by  non-sexual  budding  that 
colonies  are  normally  produced  and  a  given  locality  well  occupied 
by  the  species.  By  means  of  the  sexual  method  dispersion  is 
effected,  and  new  regions  are  occupied.  The  ova  and  sper- 
matozoa develop  in  special  gonads  (ovaries  or  testes)  derived 
in  some  species  from  the  ectoderm,  in  others  from  the  entoderm. 
The  sexual  cells  usually  escape  into  the  gastro-vascular  cavity 
and  reach  the  outside  by  way  of  the  mouth.  As  a  rule  the  sexes 
occur  in  separate  individuals.  After  fertilization  cleavage  is 
total  but  sometimes  not  equal.  A  blastula  is  formed  which  is 
often  converted  into  a  peculiar,  free-swimming,  ciliated  larva 


184 


ZOOLOGY 


.(planula),  consisting  of  a  two-layered  sac  with  no  opening. 
This  condition  may  arise  by  the  closing  up  of  an  ordinary  two- 
layered  gastrula  (as  in  Aurelia).  In  other  cases  the  entoderm 
may  be  formed  by  cells  budding  into  and  finally  lining  or  even 
filling  the  segmentation  cavity  of  an  ordinary  blastula  (Fig. 
84),  resulting  in  a  quite  similar  condition.  The  planula  after 
a  tirief  free  life  becomes  attached  by  one  pole  and  becomes 
elongated ;  a  mouth  surrounded  by  tentacles  is  formed  at  the 


— o 


FIG.  84.  Diagrams  illustrating  development  in  some  of  the  hydroid  types.  A,  blastula  in 
which  the  entoderm  (««<.)  is  produced  by  proliferation  from  ectoderm  (ect.),  B,  ciliated  planula 
formed  by  the  continuance  of  this  process.  A  split  in  the  entoderm  furnishes  the  beginning  of  the 
gastro- vascular  cavity  (g)  of  the  adult.  C,  more  mature  condition,  in  which  the  planula  has  become 
fixed:  /,  foot  or  attached  end;  o,  oral  or  free  end  at  which  the  tentacles  and  mouth  will  be  developed. 

Questions  on  the  figures. — How  does  this  blastula  differ  from  the  typical 
blastula  in  the  formation  of  entoderm  ?  What  is  a  planula  ?  Is  a  gastrula  formed  ? 
After  an  opening  forms  at  the  oral  end  what  likeness  is  there  in  the  adult  to  a  gas- 
trula? What  changes  would  C  need  to  undergo  to  become  essentially  similar  to 
Hydra? 

other.  Thus  it  assumes  the  typical  polyp  form.  In  nearly 
all  species  the  polyps  may  produce  new  individuals  by  buds 
either  from  the  wall  of  the  polyp  or  from  special  organs  (stolons, 
or  runners).  If,  when  these  are  mature,  they  separate  from  the 
parent  no  colony  is  formed.  More  commonly  the  daughters 
remain  in  association  with  the  parent.  The  medusoid  in- 
dividual,— often  of  a  very  much  simpler  type  than  that  de- 
scribed above  (§223), — may  be  produced  in  a  similar  way  from 
a  bud.  This  sexual  individual  may  degenerate  until  it  is  little 
more  than  a  case  for  the  sexual  cells.  It  usually  breaks  its 


CCELENTERATA  185 

attachment  with  the  parent  stock  and,  in  the  perfect  medusae, 
becomes  free-swimming. 

229.  Classification. — The  following  classes  of  Coelenterata  may  be  recognized. 
Class  I.  Hydrozoa  (Hydra-like  animals). — Hydrozoa  are  ccelenterates  with  two 
cell-layers  (ectoderm  and  entoderm),  between  which  there  is  a  supporting  layer 
(the  mesoglcea)  non-cellular  in  structure.  The  reproductive  cells  arise  chiefly 
from  the  ectoderm.  The  life  cycle  may  consist  of  polyps  alone  (Hydra);  or  of 
medusae  alone;  or  of  both  in  one  life  history  (Campanularia,  Pennaria,  Obelia}. 
Medusoid  forms  may  be  free  or  attached.  The  gastro-vascular  cavity  is  not 
divided  by  mesenteries.  Here  are  included  all  the  rather  scarce  fresh-water 
ccelenterates,  many  tubular  marine  forms  somewhat  similar  to  Hydra,  and  the 
much  diversified  colonies  of  the  Siphonophora  (as  the  Portuguese  Man-of-War, 
found  in  mid-ocean,  especially  in  the  region  of  the  Gulf  Stream).  See  Figs.  86,  87. 

Class  II.  Scyphozoa  (cup  animals). — Ccelenterates  in  which  the  mesenchyma 
contains  cellular  elements.  The  reproductive  cells  arise  from  the  entoderm  and 
escape  into  the  digestive  cavity.  Chiefly  medusoid  forms,  though  in  some  the 
bell-form  alternates  with  a  polyp  stage.  Types:  Aurelia  and  the  larger  jelly- 
fishes.  The  majority  of  the  Scyphozoa  swim  on  the  surface  of  the  ocean;  some  are 
found  at  considerable  depths.  Many  of  them  are  very  large  and  handsome.  An 
especially  interesting  fact  in  connection  with  the  development  of  such  a  type  as 
Aurelia  is  that  its  polyp  (known  as  the  Scyphistoma}  is  intermediate  in  its  charac- 
teristics between  the  polyps  of  the  Hydrozoa  and  those  of  the  Actinozoa.  The 
Scyphistoma  has  four  ridges  which  partly  separate  the  gastro-vascular  cavity  as  do 
the  mesenteries  in  the  Actinozoa. 

Class  III.  Actinozoa  (ray  animals). — Coelenterates  with  only  the  polyp  form. 
Cells  in  the  mesenchyma.  There  is  a  well-developed  ectodermic  gullet  (stomo- 
daeum).  The  gastro-vascular  cavity  is  more  or  less  completely  divided  into 
chambers  by  mesenteries.  Sexual  cells  entodermal.  A  skeleton  of  calcareous  or 
horny  material  often  present. 

Types:  Sea-anemones;  sea-fans  and  corals.  The  sea-anemones  or  sea-roses 
are  common  on  rocks  and  other  objects  just  below  low-water  mark.  Though 
attached,  they  have  some  power  of  gradually  changing  their  position.  Species 
of  sea-anemones  are  known  in  which  the  individuals  are  as  much  as  two  feet  in 
diameter,  though  polyps  of  the  colonial  forms  are  usually  very  small. 

Class  IV.  Ctenophora  (" comb-bearers"}. ---The  Ctenophora  are  free-swimming, 
pear-shaped  jelly-fishes,  never  occurring  in  colonies,  and  not  associated  with  a 
polyp  stage.  They  bear  eight  meridional  plates  supplied  with  transverse  rows  of 
cilia,  which  function  as  locomotor  and  possibly  as  respiratory  organs,  and  suggest 
the  name  of  the  group.  There  is  a  well-developed  stomodaeum.  The  gastro- 
vascular  canal  branches  from  this  and  is  much  divided,  one  division  lying  under 
each  row  of  combs.  There  are  two  small  aboral  openings  to  the  digestive  canal 
known  as  excretory  pores.  The  mesenchyma  is  well  developed.  There  are  no 
nettling  cells  as  in  the  other  ccelenterates;  but  glue-cells  with  similar  structure  and 
functions  are  found.  (Some  students  of  the  Ctenophora  place  them  in  a  separate 
phylum.) 

230.  Ecology,    etc. — The    food    of    Ccelenterates    consists 
largely  of  organic  debris  broken  up  by  the  waves,  and  of  small 


i86 


ZOOLOGY 


animals  and  plants  captured  by  the  tentacles.  The  attached 
forms  flourish  best  in  the  comparatively  shallow  water  near  the 
shore.  Food  is  especially  abundant  in  such  regions  and  hence 
the  passive  animals  are  more  successful  here  than  elsewhere. 
Hydractina  (Fig.  85)  and  even  the  sea-anemone  form  interesting 
partnerships  with  the  hermit-crab.  The  polyps  cover  up  the 
shell  occupied  by  the  crab,  thus  concealing  it  from  its  enemies 
and  its  prey.  In  return  the  polyps  doubtless  profit  by  a  share 

FIG.  85. 


FIG.  85.  Hydractina  echinata,  after  Hincks.  c.  the  cctnosarc,  forming  an  incrustation  over  the 
object  on  which  it  lives;  «,  nutritive  polyps;  r,  reproductive  polyps,  bearing  buds  in  which  are  ova; 
/,  tentacles. 

Questions  on  the  figure. — How  many  types  of  individuals  seem  to  be  repre- 
sented? What  evidence  of  budding  do  you  see  in  the  species?  What  is  the  cceno- 
sarc?  What  is  its  nature  in  Hydractina  ?  What  can  you  find  concerning  the  habits 
of  the  members  of  the  genus?  How  does  this  colony  compare  with  that  in  Fig.  86? 

of  the  food  broken  to  pieces  by  the  crab,  as  well  as  by  the  change 
of  place  as  the  crab  moves  about  in  search  of  food.  Some  anem- 
ones have  living  algae  in  their  entoderm  cells  which  seem  to 
help  supply  the  animal  with  oxygen  in  return  for  foods  of  other 
kinds. 

Nearly  all  the  ccelenterates  are  marine.     A  few  species  of 
Hydra,  one  or  two  partly  parasitic  types,  and  a  few  fresh- water 


CCELENTERATA 


I87 


medusoids  are  the  only  known  exceptions.  Cordylophora,  a 
colonial  polyp,  is  found  in  brackish  tidal  waters,  and  has  been 
discovered  in  inland  rivers.  It  seems  to  be  a  species  in  process 
of  adjustment  to  fresh- water  conditions. 

FIG.  86. 


FIG.  86.     Physalia,  the  Portuguese  Man-of-War.     After  Agassiz. 

Questions  on  the  figure. — For  what  is  this  animal  remarkable?  To  what 
group  of  coelenterates  does  it  belong?  Compare  Huxley's  figure  of  the  same 
animal  (see  Parker  and  Haswell's  Zoology,  Vol.  I,  p.  152,  and  other  reference 
texts).  What  various  types  of  polyps  are  represented  in  the  colony?  Compare 
with  Fig.  87. 

The  jelly-fish  are  used  as  food  by  fishes  and  whales,  though 
they  cannot  contain  much  solid  matter.  The  group  has  little 
economic  value.  The  red  coral  of  the  Mediterranean,  of  very 
slow  growth,  is  used  in  making  jewelry.  The  reefrforniing  corals 


i88 


ZOOLOGY 


have  changed  the  land  contours  by  additions  of  marginal  reefs. 
Much  of  the  limestone  of  the  crust  of  the  earth  is  from  corals. 

FIG.  87. 


s.b 


r.z: 


FIG.  87.  A  very  diagrammatic  and  generalized  illustration  of  a  complex  coelenterate  colony. 
The  shaded  portion  represents  the  gastro-vascular  cavity.  The  light  portion,  the  body  tissues. 
b.,  a  bell-like  individual  developed  into  an  air-bladder;  m.t  mouth;  n.z.,  nutritive  individual;  p.*.. 
protective  individual;  r.z.1,  r.z.*,  r.  z.*f  different  types  of  reproductive  individuals;  s.b.,  swimming 
bell;  /,  tentacles,  which  are  sensory  and  protective  structures.  After  Lang. 

Questions  on  the  figure. — What  is  meant  by  "generalized"  above?  How  does 
such  a  polymorphic  colony  as  this  differ  from  a  highly  organized  individual? 
In  what  respect  is  it  similar  to  an  individual  ?  What  is  the  function  of  the  gastro- 
vascular  system?  What  is  the  gain  in  its  wide  distribution  through  the  colony? 
How  do  the  Siphonophora  differ  from  the  other  colonial  coelenterates  ? 


CCELENTERATA  189 

Many  interesting  experiments  have  been  performed  on  mem- 
bers of  this  group  illustrating  the  power  of  regenerating  lost 
parts.  Many  of  the  polyps  have  been  shown  to  have  this  power 
and  even  the  medusa  may  become  perfect  animals  again  after 
having  lost  very  considerable  portions  of  their  structure.  Hydra, 
one  of  the  simplest  members  of  the  group,  is  most  famous  for 
its  power  of  regaining  its  original  form,  no  matter  to  what  sort 
of  mutilation  it  has  been  subjected.  As  long  as  there  is  a  piece 
of  the  trunk  of  appreciable  size  containing  both  ectoderm  and 
entoderm  it  may  regenerate  the  whole  animal, — stalk,  mouth, 
tentacles,  and  all, — under  favorable  conditions. 

Nothing  about  the  ccelenterates  is  more  interesting  to  the 
zoologist  than  the  way  in  which  the  individuals  in  the  poly- 
morphic colonies  (as  in  the  Siphonophora)  come  to  do  the  work 
done  by  special  organs  in  the  higher  Metazoa. 

231.  Supplementary  Studies,  for  field  and  library. 

1.  Make  a  list  of  all  the  places  where  Hydra  may  be  found 
in  your  locality. 

2 .  Can  you  find  an  account  of  any  other  fresh-water  Ccelen- 
terata  ? 

3.  What  facts  can  you  find  concerning  the  power  of  regen- 
eration in  Hydra  or  other  coelente rates  ? 

4.  Coral  reefs:  kinds  and  mode  of  formation.     Conditions 
of  life  necessary  to  the  reef -forming  corals. 

5.  Polyp  colonies.     Show,  by  reference  to  all  the  specimens 
and  figures  you  can  find,  where  the  newest  bud  appears  and 
how  this  helps  determine  the  shape  of  the- colony. 

6.  Polymorphism  and  division  of  labor  in  polyp  colonies. 

7.  Corals  in  geological  time. 

8.  Sense  organs  among  ccelenterates. 

9.  Alternation  of  generation  in  Obelia.     In  Amelia. 

10.  The  symmetry  of  the  ccelenterates. 

11.  The  structure,  position  and  uses  of  the  nettling  cells  in 
the  phylum. 

12.  Study  the  polyp  of  Aurelia  (Scypkistoma)  from  descrip- 
tions and  cuts,  and  show  in  what  respects  it  seems  to  stand 
intermediate  between  the  Hydrozoa  and  the  Actinozoa. 


CHAPTER  XIII 

UNSEGMENTED  WORMS  (FLAT-WORMS,  THREAD-WORMS, 
ROTIFERS,  POLYZOA,  ETC.) 

232.  It  seems  desirable,  for  the  sake  of  convenience  and 
in  order  to  prevent  a  confusing  array  of  details,  to  embrace 
under  this  head  a  number  of  groups  of  animals  which  do  not 
have  very  much  in  common  except  their  place  of  uncertainty 
in  the  animal  kingdom.     They  are  not  to  be  considered  as 
forming  a  phylum  of  animals,  although  in  the  past  they  have 
often  been  included  by  authors  with  the  Annulata  (Chapter 
XV)  under  the  head  of  Vermes.     There  is  abundant  evidence 
indeed  to  enable  one  to  believe  that  four  or  five  distinct  phyla 
are   here   included.     Some    of   these    groups,    however,    have 
members  which  bear  more  or  less  striking  resemblances  to 
animals  belonging  to  the  recognized  phyla,  especially  to  embry- 
onic stages  of  them.     These  facts  render  them  of  the  greatest 
possible  interest  to  the  zoologist,  because  they  furnish  grounds 
for  the  hope  that,   through  the  study  of  this  heterogeneous 
assemblage,  the  origin  and  kinships  of  the  other  phyla  may 
be  made  more  clear.     The  same  facts  make  them  unfit  objects 
for  extended  study  in  elementary  classes. 

233.  Points   of   General  Resemblance. — In  external  form 
these  animals  differ  -very  greatly.     They  may  vary  from   a 
cylindrical  or  even  a  globular  form  to  a  thin  ribbon-shape. 
They  agree  for  the  most  part,   however,   in  having  a  main 
axis  which  in  the  free-swimming  forms  is  usually  horizontal 
in  position,  the  anterior  end  of  which  is  structurally  distin- 
guishable  from    the    posterior.     There   is    usually    a    distinct 
bilateral  symmetry  (see    §120)    which   takes   the  place  of  the 
radial  symmetry  found  in  the  Ccelenterates.     In  some  types  of 
the 'Ccelenterates  there    are    certain    suggestions    of    bilateral 
symmetry  but  never  to  the  complete  exclusion  of  the  radial. 
For  the  first  time  is  found  an  assemblage  of  multi-cellular 

190 


UNSEGMENTED    WORMS  IQI 

animals  whose  individuals  move  with  one  end  continually 
foremost  and  one  of  the  body  surfaces  continually  up  and 
the  other  down.  This  is  a  distinct  gain  in  organization  and 
accompanies  a  more  active  life.  The  Polyzoa  are  attached 
in  adult  life  and  have  lost  this  symmetry,  and  many  of  the 
Rotifers,  while  having  definite  anterior  and  posterior  ends, 
have  lost  their  right-left  symmetry  in  part,  but  the  embryonic 
stages  of  these  are  in  many  respects  similar  to  the  more  typical 
forms.  By  saying  that  these  animals  are  unsegmented  it 
is  meant  that  in  a  distinct  individual  there  is  not  usually  a 
linear  series  of  equivalent  body-parts  or  metameres.  There 
are  however  several  types  which  reproduce  new  individuals 
by  transverse  division  ("fission").  These  new  individuals 
may  remain  together,  temporarily  at  least,  in  a  chain,  as  in 
Microstomum  (Fig.  91)  or  the  tape- worm  (Fig.  93),  form- 
ing a  strobila.  In  this  condition  there  is  a  repetition  of  all 
the  essential  organs  in  each  of  the  "segments."  Some  authors 
regard  this  process  of  strobilation  as  the  condition  from  which 
the  ordinary  segmentation,  as  seen  in  the  Annulata,  has  arisen, 
by  the  adhesion  and  gradual  differentiation  and  coordination 
of  the  originally  similar  individuals. 

The  animals  of  these  groups  agree  in  the  fact  that  the  third 
or  mesodermal  layer  of  tissue  becomes  more  important  than 
it  is  among  the  Crelenterates.  They  are  therefore  triploblastic 
animals.  In  addition  to  this  the  mesoderm  often,  though 
not  universally,  splits,  forming  a  ccelom  or  body  cavity  (§58) 
wholly  separate  from  the  digestive  tract.  The  ccelom  is  lined 
with  mesoderm.  All  the  animal  phyla  above  the  Ccelenterates 
possess  this  character  in  some  measure  and  on  this  account 
are  called  Ccelomata. 

These  animals  further  agree  with  those  above  them  in  the 
scale  of  development  in  possessing  a  system  of  excretory  tubules 
which  connect  the  ccelom,  or  the  mesodermal  tissue  if  there 
is  no  ccelom,  with  the  outside  world.  This  system  is  believed 
to  get  rid  of  nitrogenous  wastes. 

234.  Laboratory  Exercises. — An  extended  laboratory  study  of  these  groups  is 
not  desirable,  yet  the  teacher  should  secure  enough  material  representing  the 
various  included  phyla  to  enable  the  student  to  justify  the  separation  of  these 


IQ2  ZOOLOGY 

uncertain  forms  from  the  more  exactly  denned  phyla,  and  to  show  him  how 
ill-defined  is  the  assemblage  which  we  have  thus  brought  together.  The  Tape- 
worm of  man  may  sometimes  be  secured  from  physicians,  and  other  species  of 
worms  are  found  not  infrequently  as  intestinal  parasites  in  cats,  dogs,  or  other  animals 
dissected  in  the  laboratory.  The  general  form,  the  method  of  attachment  to  the 
host,  the  progressive  development  of  the  proglottides  or  "segments,"  and  the 
difference  between  these  segments  and  those  of  the  earth-worm  should  be  noted. 
Permanent  whole  mounts  of  a  mature  proglottis  may  be  made,  showing  the 
embryos  in  the  uterus.  Demonstrations  of  the  structure  of  the  proglottis  may  be 
given  by  properly  prepared  transverse  sections,  if  the  equipment  and  time  allow. 

An  hour's  work  may  profitably  be  devoted  to  the  study  of  some  one  or  more  of 
the  common  Rotifers,  which  may  be  found  in  water  taken  from  the  stagnant  pools 
in  which  there  is  much  decaying  matter.  They  are  microscopic  animals  and  are  to 
be  recognized  by  the  possession  of  discs  at  the  anterior  end,  which  present  the 
appearance  of  rotating  wheels  because  of  a  rhythmic  action  of  the  cilia.  Make 
sketches  showing  the  change  of  shape  which  the  animal  undergoes.  How  is  the 
change  effected?  How  is  locomotion  accomplished?  What  evidences  have  you 
of  its  ability  to  receive  stimuli  and  to  respond  to  them?  How  does  it  get  food? 
Can  you  trace  the  digestive  tract  in  the  body  of  the  animal?  Notice  the  contract- 
ing object  just  back  of  the  mouth.  What  conclusions  do  you  reach  as  to  its  func- 
tion? Give  your  evidences.  Verify  by  consulting  some  textbook.  Can  you 
prove  from  what  you  see  that  this  is  not  a  single-celled  animal  like  Stentor?  The 
student  should  be  cautioned  against  taking  these  specimens  as  closely  typical  of 
the  whole  group  of  Rotifers,  since  there  is  very  great  variety  of  form  among  them. 

Planarians  often  appear  in  the  laboratory  in  water  containing  an  abundance 
of  decomposing  organic  matter,  taken  from  ponds  and  foul  streams.  The  most 
important  points  to  be  noticed  are  their  general  form,  the  method  of  locomotion, 
sensitiveness  to  stimuli,  and  life  habits.  Non-sexual  reproduction  by  fission  is 
frequent  among  them. 

The  Polyzoa  occur  as  tufts  of  many  minute  animals  in  colonies  attached  to 
objects  in  the  water.  Plumatella  is  a  rather  common  fresh-water  form  and  makes 
a  beautiful  demonstration  to  illustrate  the  ordinary  physiological  processes,  as 
motion,  feeding,  the  action  of  the  digestive  tract  in  churning  the  food,  sensitive- 
ness to  stimulus  and  the  like.  Schools  near  the  sea-shore  will  find  an  abundance 
of  marine  material  for  the  comparison  of  the  colonial  forms  of  different  species 
of  Polyzoa,  since  they  are  more  common  in  salt  than  in  fresh  water. 

235.  Classification  and  Description.  Phylum  IV: — Platyhelminthes  (Flat- 
worms}. — In  the  worms  of  this  phylum  the  body  is  flattened  or  compressed  in  a 
dorso-ventral  direction,  and  from  this  fact  the  name  is  given.  They  are  soft- 
bodied  animals  without  any  true  skeleton.  There  is  no  body  cavity  and  no  true 
blood-vascular  system.  The  space  which  would  be  given  to  such  structures  is 
filled  with  a  spongy  connective  tissue.  Through  this  body-mass  run  the  minute 
tubes  of  the  excretory  or  water-vascular  system  (Fig.  88,  ex.},  often  terminating 
internally  in  special  cells  (flame  cells,  Fig.  90).  These  tubes  have  .external  pores. 
By  means  of  this  system  of  organs  waste  products,  probably  of  a  nitrogenous 
nature,  are  eliminated  from  the  tissues.  The  digestive  tract  may  be  wholly  want- 
ing as  in  the  Cestodes,  or  a  simple  or  forked  sac,  or  a  central  sac  with  lateral 
branches.  It  is  blind,  i.e.,  has  only  the  oral  opening.  In  the  more  complicated 
types  of  stomach  the  much-branched  sac  serves  the  function  of  carrying  the  digested 


UNSEGMENTED    WORMS 


193 


food  to  all  parts  of  the  body.  Many  of  these  forms  are  parasitic  and  in  conse- 
quence the  organs  referred  to  are  often  very  much  simplified  and  degenerate.  The 
digestive  tract,  for  example,  may  be  entirely  lost.  Reproduction  by  transverse 
division  is  not  uncommon.  By  this  method  strobilae  or  chains  of  more  or  less 
closely  connected  individuals  occur  (Figs.  91,  93).  The  sexual  organs  are  ex- 
ceedingly complex,  particularly  in  the  parasitic  members  of  the  group  (Fig.  94). 
The  development  is  in  some  instances  direct,  in  others  indirect.  The  principal 
classes  are  the  Turbellaria,  Trematodes  and  Cestodes. 


FIG. 


FIG.  88.  Diagram  of  a  Turbellarian,  showing  the  general- arrangement  of  the  nervous  structures 
and  one  of  the  modes  of  occurrence  of  the  excretory  tubules,  which  in  this  case  open  separately  into 
the  pharynx,  on  the  ventral  side  of  the  animal,  b,  brain;  «,  eye-spots;  ex,  excretory  canals  con 
sisting  of  a  transverse  portion  passing  from  the  mouth  toward  the  dorsal  side  (see  also  Fig.  89), 
and  longitudinal  tubes  which  branch  into  the  capillary  vessels  terminating  in/,  the  flame  cells;  I.e., 
lateral  nerve  cords;  m,  mouth. 

Questions  on  the  figure. — Compare  this  figure  with  the  next  and  identify  the 
structures  shown  in  both.  What  other  positions  of  the  mouth  do  you  discover  in 
the  Turbellaria,  as  figured  in  reference  texts?  What  other  arrangement  of  the 
excretory  canals  and  pores? 

Class  I.  Turbellaria  (Planarians,  etc.}. — These  are  mostly  small  non-parasitic 
Platyhelminthes  with  a  ciliated  ectoderm.  They  are  chiefly  aquatic  and  are  car- 
nivorous. The  ventral  mouth  may  be  anterior,  posterior,  or  median  in  position. 
It  opens  into  a  muscular  eversible  pharynx,  which  may  be  used  to  assist  in  locomo- 
tion as  well  as  in  capturing  food.  The  digestive  tract  may  be  simple  or  very  much 
branched.  The  brain  consists  of  a  pair  of  ganglia  in  the  anterior  region.  From 
the  brain  lateral  nerve  cords  pass  backward  through  the  body.  The  excretory 
organs  (Figs.  88,  89)  usually  consist  of  two  or  more  longitudinal  tubes  which  open 
on  the  exterior  separately  or  by  a  common  orifice.  The  position  of  the  opening 
13 


194 


ZOOLOGY 


FIG.  89. 


FIG.  90. 


FIG.  89.  Diagram  of  transverse  section  of  a  Turbellarian  through  the  region  of  the  mouth. 
d.m.,  dermo-muscular  wall  containing  longitudinal  fibres;  ex,  excretory  system;  /,  flame  cells;  g, 
gut;  I.e.,  lateral  nerve  cord;  m,  mouth;  m./.,  muscle  fibres;  ph.,  pharynx;  t,  testis;  «,  uterus;  y, 
yolk  glands. 

Questions  on  the  figure. — Determine  with  care  the  relation  of  this  to  the 
preceding  diagram  and  identify  the  common  structures.  What  new  structures 
are  represented  here?  What  would  be  their  position  in  the  former  figure?  The 
great  range  in  position  of  the  muscle  fibres  and  the  spongy  character  of  the  body 
contribute  to  what  powers? 

PIG.  90.  Diagram  of  flame  cell,  the  internal  terminus  of  the  excretory  tubules,  c,  cilia  lining 
the  tubule;/,  special  cilia  constituting  the  flame;  n,  nucleus  of  flame  cell;  p,  cell  processes;  v,  vacuole 
or  cavity  in  cell  communicating  with  the  capillary  tubules  (I). 

Questions  on  the  figure. — What  is  the  function  of  the  cell  itself  ?     Of  the  flame  ? 


FIG.  91.  Diagrammatic  sagittal  section  of  Microstomum,  showing  a  chain  of  four  zooids 
produced  by  fission,  b,  brain  of  the  original  zooid  (the  exponents  indicating  corresponding  struc- 
tures of  the  more  recently  formed  zo6ids);  c,  ciliated  pit;  d,  dissepiments  indicating  different  stages 
in  the  separation  of  the  zooids;  e,  eye-spot;  «wf,  entoderm;g,  gut;  gl.,  glandular  cells  about  the  mouth; 
m,  mouth  of  the  original  worm. 

Questions  on  the  figure. — What  various  evidences  can  be  found  of  the  relative 
age  of  the  zooids?  Is  the  mouth  formed  apparently  from  entoderm  or  ectoderm? 
Is  the  gut  a  blind  sac?  What  incidents  seem  necessary  when  this  chain  separates 
at  the  oldest  plane  of  division,  and  forms  two  chains,  in  order  that  each  may  be 
like  the  parent?  How  is  this  like  segmentation  in  annulates  (see  Fig.  101)  ?  How 
unlike? 


UNSEGMENTED    WORMS  1 95 

varies  very  much  in  the  different  orders.  The  tubules  are  much  branched 
interiorly  and  penetrate  the  soft  tissues  of  the  body  as  minute  capillaries  with 
thin  walls.  They  terminate  in  cells  of  special  structure  which  are  excretory  in 
function.  A  group  of  cilia  (the  flame,  Fig.  90,  /)  helps  in  creating  a  current  in  the 
capillary  tubes.  The  lining  of  the  tube  may  also  be  supplied  with  cilia.  Repro- 
duction is  by  division  (fission,  Fig.  91)  or  by  eggs  and  sperm.  The  Turbellaria 
have  remarkable  powers  of  regenerating  lost  portions.  Experiments  show  that 
very  small  portions  of  an  individual  will,  under  favorable  conditions,  reproduce  all 
the  parts  of  a  complete  animal.  In  habit  they  may  be  terrestrial,  fresh-water  or 
marine.  They  vary  in  size  from  microscopic  fresh-water  forms  to  a  length  of  six 
inches  or  more  in  the  case  of  the  marine  and  land  types  (Figs.  88-91). 

Class  II.  Trematoda. — The  Trematodes  are  small,  usually  parasitic,  Platyhel- 
minthes.  The  ectoderm  is  provided  with  a  protective  "cuticle"  and  is  conse- 
quently destitute  of  cilia.  They  possess  a  well-developed  and  often  much- 
branched  digestive  sac,  which  has  only  one  opening — the  mouth.  Usually  one  or 
more  sucking  discs  are  present.  By  means  of  these  the  parasite  attaches  itself  to 
the  host.  The  nervous  and  excretory  systems  are  similar  in  general  to  those  of  the 
Turbellaria,  but  are  somewhat  better  developed  and  more  complex.  In  those 
members  of  the  class  which  are  external  parasites  there  is  usually  no  metamorphosis 
in  the  development.  In  the  internal  parasites,  as  the  Liver-fluke  of  the  Sheep, 
there  is  a  most  complicated  metamorphosis  coupled  with  an  alternation  of  sexual 
and  non-sexual  generation  (see  §223).  A  Liver-fluke  (Distomum  hepaticum)  is 
found  in  the  bile  ducts  of  the  liver  of  the  sheep,  where  it  gives  rise  to  a  much- 
dreaded  disease — "liver  rot."  The  eggs  which  are  formed,  fertilized  and  pass 
through  the  early  stages  of  cleavage  here,  pass  out  of  the  bile  ducts  to  the  intestine 
and  thence  to  the  exterior.  If  the  larva  reaches  water  it  develops  into  a  free- 
swimming  larva  (Fig.  92,  C),  which  to  insure  further  development  must  bore  into 
the  tissues  of  a  particular  pond-snail  (Limnaa  truncatula).  It  there  develops  into 
a  kind  of  sac  (sporocysf)  from  the  inner  cells  of  which  special  cells  are  budded 
(Fig.  92,  £).  These  cells  have  the  power  of  developing  into  embryos  of  a  second 
generation  by  cell  division — that  is  to  say,  non-sexually.  Several  such  non-sexual 
reproductions  may  occur  in  the  body  of  the  snail  (Fig.  92,  +).  These  later  genera- 
tions of  larvae  pass,  often  by  the  death  of  the  snail,  into  the  water,  whence  they  may 
enter  the  alimentary  tract  of  the  sheep  in  drinking.  The  larvae  find  their  way  to 
the  liver  and  develop  there  again  into  the  adult  fluke.  It  is  evident  that  such  a 
form  must  have  immense  powers  of  reproduction,  when  it  is  considered  that  the 
reproduction  takes  place  at  several  points  in  the  life  cycle  (Fig.  90,  +  *).  This 
may  be  seen  to  be  a  necessity  to  compensate  for  the  great  loss  of  life  involved  in 
changing  from  host  to  host.  It  is  said  that  a  single  fluke  may  produce  half  a  million 
eggs.  Each  of  these  which  succeeds  in  reaching  the  host  snail  may  produce 
thousands  of  the  last  generation  of  asexual  individuals,  and  yet  the  numbers  of  the 
species  probably  do  not  increase.  The  disease  is  prevalent  only  in  those  countries 
where  this  species  of  Limncza  occurs.  It  is  much  worse  in  wet  years.  Millions  of 
sheep  have  died  in  England  alone,  in  a  single  year,  from  the  attacks  of  this  parasite. 
Trematode  parasites  are  common  among  animals  and  frequent  most  diverse  organs. 
As  compared  with  the  Turbellaria,  the  Trematodes  have  lost  their  eye-spots,  have 
less  well  developed  sense  organs  and  central  nervous  systems,  and  have  highly 
elaborate  reproductive  organs  and  metamorphosis.  These  facts  are  related  to  the 
parasitic  habit. 


ig6 


FIG.  92.  A  series  of  diagrams  illustrating  the  life  cycle  in  the  LIVER-FLUKE  (Distomum).  After 
Thomas,  Leuckart,  and  others.  A,  egg  in  its  case;  B,  early  embryo,  still  in  case;  C,  free-swim- 
ming  ciliated  embryo ;  D,  same  after  encysting  in  tissues  of  snail  (sporocysf) ;  E,  sporocyst  at  later 
stage  producing  by  internal,  non-sexual  processes  new  sporocysts,  and  redia.  (r)  which  break  from  the 
sporocyst  and  lead  an  independent  life  of  their  own  in  the  tissues  of  the  snail;  F,  a  mature  redia 
producing  within  itself  new  generations  of  rediae,  and  a  new  type  of  larva,  cercaria,  which  escape 
by  a  birth-pore  (b.p.)  and  make  their  way  into  the  water;  G,  cercaria;  H,  same  after  losing  its 
tail  and  becoming  encysted;  7,  the  young  fluke  in  the  liver  of  the  sheep,  where  it  becomes  sexually 
mature  and  produces  perhaps  500,000  new  eggs,  b,  brain;  b.p.,  birth-pore;  c,  cercaria;  c.m.t  cell 
masses, — embryos  formed  non-sexually  within  sporocysts  and  rediae;  e,  eye-spots;  ex.,  excretory 
tubules  and  pore  (only  the  posterior  portion  shows);  g,  gut;  m,  mouth;  ph,  pharynx;  r,  redia;  s, 
suckers;  sc,  sporocyst;  +,  stages  in  which  non-sexual  reproduction  occurs;  *,  stage  at  which  sexual 
reproduction  occurs. 

Questions  on  the  figures. — In  which  stages  are  eyespots  found?  Number  and 
position  of  the  suckers?  In  which  stages  found?  What  is  the  result  of  increasing 
the  points  at  which  reproduction  occurs  in  the  cycle?  Is  this  a  combination  of 
metamorphosis  and  alternation  of  generation?  Your  reasons  for  your  answer? 
Compare  this  with  the  life  history  of  the  tape-worm.  Note  the  encysted  stage  by 
which  it  passes  from  water  to  its  host  in  each  instance. 


UNSEGMENTED    WORMS 


IQ7 


400 


-doom CO3 


D 


PIG.  93.  Diagram  showing  some  stages  in  the  life  history  of  the  Tapeworm  (Tcenia).  A, 
Cysticercus  or  Bladderworm  stage,  before  the  "head"  protrudes  from  the  bladder;  B,  same,  later 
stage;  C,  Strobila,  or  chain  of  proglottides,  many  being  omitted;  D,  embryo,  such  as  fill  the  uterus 
of  the  mature  proglottides.  It  is  protected  by  a  shell,  b,  bladder;  ex.,  excretory  canals;  g,  genital 
pore;  A,  head  or  scolex  provided  with  hooks  and  suckers  (5);  «,  uterus  in  a  mature  posterior  pro- 
glottis;  s,  zone  of  budding  or  segment  formation.  The  numerals  show  the  approximate  number  of 
the  segments,  reckoning  from  the  front.  Not  more  than  5  per  cent,  of  real  length  of  the  chain 
is  represented. 

Questions  on  the  figures. — What  arguments  do  you  find  from  the  figure  for 
considering  the  strobila  an  individual?  What  for  considering  it  a  colony? 
Where  does  non-sexual  reproduction  occur?  Where  sexual?  Seek  figures  of 
stages  between  D  and  A  in  the  reference  books. 


198 


ZOOLOGY 


Class  III.  Cestodes  (Tape-worm,  etc.). — The  Cestodes  are  internal  parasites 
having  a  complicated  life  history  usually  involving  two  hosts.  In  the  tissues  of 
the  first  host  occurs  the  "bladder- worm,"  Cysticercus,  or  embryonic  stage  (Fig.  93, 
A) ;  in  the  intestine  of  a  second  host  the  strobila  or  adult  tape-worm  (Fig.  93,  Q  is 
found.  The  adult  form  has  no  mouth  or  digestive  tract,  the  animal  taking  its 
food  by  absorption  of  the  digested  material  in  which  it  is  bathed.  The  anterior 
end  is  supplied  with  hooks  or  suckers  by  means  of  which  it  attaches  itself  to  the 
intestinal  wall.  Just  behind  this  "head"  is  a  region  in  which  transverse  division 


fl  3. 


FIG.  94.  Diagiam  of  a  sexually  mature  proglottis  of  Toenia.  A,  anterior  end;  e,  embryos; 
ex.,  excretory  canals;  g.p.,  genital  pore;  ov.,  ovaries  (paired);  r.s.,  receptaculum  seminis;  5.?.,  shell 
gland;  t,  testes;  ut.,  uterus  filled  with  embryos;  v,  vagina;  v.d.t  vas  deferens;  y.g.,  yolk  gland. 

Questions  on  the  figure. — Why  is  self-fertilization  possible  in  tapeworm? 
What  is  the  function  of  the  various  portions  of  the  reproductive  apparatus? 
Trace  the  following  steps  and  indicate  where  each  incident  happens:  formation  of 
eggs  and  sperm;  passage  of  sperm  to  vas  deferens  and  into  vagina;  storing  of  sperm 
in  receptaculum  seminis;  fertilization  in  the  oviduct;  addition  of  yolk;  ovum 
covered  with  shell  secretion;  passage  into  uterus  where  development  proceeds. 

(Fig.  93,  z;  and  §126)  is  continually  taking  place.  This  results  in  the  continuous 
formation  of  new  segments  or  proglottides,  the  older  ones  being  pushed  further  from 
the  head  by  those  newly  formed.  Each  proglottis  becomes  in  time  a  sexually 
mature  hermaphrodite  individual.  All  stages  of  sexual  maturity  are  found  in  one 
strobila  or  colony,  the  posterior  individuals  being  most  mature.  At  the  posterior 
end  of  an  old  colony  the  proglottides  (Figs.  93,  94)  are  rilled  with  the  developing 


UNSEGMENTED    WORMS  !<;<) 

embryos,  and  on  breaking  away  from  the  chain  these  brood  cases  pass  with  the 
fecal  matter  from  the  intestine.  In  this  way  it  becomes  possible  for  the  embryos 
to  find  the  way  into  a  new  host.  On  being  swallowed  by  some  suitable  animal 
they  break  from  their  cysts,  bore  through  the  wall  of  the  digestive  tract  into  the 
tissues.  Here  they  grow,  become  encysted  and  at  this  stage  develop,  in  antici- 
pation of  the  needs  of  the  adult  worm,  the  head  or  scolex  which  remains  attached 
to  the  bladder-like  cyst  (Fig.  93,  A,  B).  Development  stops  at  this  point  unless 
the  flesh  of  this  host  is  eaten  by  some  other  animal.  When  this  happens  the  blad- 
der is  thrown  off,  the  head  becomes  attached  to  the  wall  of  the  intestine  of  the 
carnivorous  host,  and  the  active  formation  of  the  chain  of  proglottides  begins 
again. 

The  more  common  tape- worms  of  man  are  Tania  solium  and  Tcenia  saginata. 
The  former  is  more  common  in  Europe  and  is  received  into  the  system  by  eating 
the  raw  flesh  of  the  pig,  in  which  the  bladder-worm  stage  occurs.  The  latter  is 
obtained  chiefly  from  beef  and  is  more  common  in  America.  Only  by  adequate 
cooking  is  the  danger  of  infection  removed.  The  American  habit  of  eating  beef 
rare  "contributes  to  the  spread  of  the  pest.  Other  tapeworms  infest,  as  their 
double  host,  the  dog  and  the  rabbit;  man  and  fish;  the  cat  and  the  mouse;  the 
shark  and  other  fishes. 

The  excretory  system  is  a  pair  of  continuous  lateral  tubes  with  transverse 
connections  in  the  various  proglottides  (Fig.  94,  ex}.  The  nervous  system  in  the 
adult  tapeworm  includes  a  rather  complex  series  of  loops  containing  nerve-cells, 
in  the  scolex,  with  right  and  left  lateral  lines  of  nervous  tissue  running  the  length 
of  the  strobila.  There  are  numerous  longitudinal,  transverse  (circular),  and  dorso- 
ventral  muscle  fibres  passing  through  the  spongy  tissue  of  the  worm.  There  is 
a  well-developed  external  cuticle  which  helps  protect  the  animal  from  the  action 
of  the  digestive  juices  of  the  host. 

Phylum  V: — Nemathelminthes  (Round-  or  Thread-worms). — Nemathelminthes 
are  elongated,  cylindrical  forms  which  taper  at  the  ends.  The  body  is  covered  by 
a  dense  cuticle.  Many  are  aquatic,  but  some  are  parasitic  at  least  during  a  part 
of  their  life.  An  alimentary  tract  is  present  and  has  both  a  mouth  and  an  anus. 
There  is  a  ccelom  which  is  not  divided  into  chambers  and  contains  a  fluid  without 
corpuscles.  There  is  no  circulatory  system  other  than  this.  There  are  no  special 
respiratory  organs.  The  central  nervous  system  consists  of  a  ring  around  the 
esophagus.  This  contains  some  nerve  cells.  From  this  ring  nerves  arise  at 
various  points  and  pass  both  forward  and  backward.  The  chief  posterior  nerve  is 
ventral,  but  there  may  be  also  dorsal  and  lateral  ones.  The  sexes  are  usually 
separate.  Development  is  sometimes  direct,  sometimes  indirect.  The  best- 
known  representatives  are  the  round-worms  (Ascaris),  different  species  of  which 
are  found  in  the  intestine  of  man,  of  the  pig,  and  of  the  horse;  vinegar- "eels"; 
trichina;  and  numerous  free-swimming  forms. 

Trichinella  is  one  of  the  most  dangerous  of  the  nematode  parasites.  The 
sexually  mature  worm  occurs  in  the  intestine  of  the  rat,  the  pig,  man,  or  other 
mammal.  The  young  are  retained  by  the  mother  in  the  uterus  until  well  developed. 
When  born  the  young  bore  through  the  wall  of  the  intestine  of  the  host  and  make 
ther  way  to  the  muscles,  where  they  become  encysted  and  cause  degeneration 
of  he  muscle  fibres  and  often  other  acute  symptoms  of  the  disease  known  as 
trichinosis.  The  larvae  remain  in  their  cysts  indefinitely  or  until  the  death  of 
their  host.  For  further  development  the  flesh  must  be  eaten.  In  the  intestine 
of  the  new  host  where  the  cyst  is  dissolved  the  adult  condition  is  quickly  reached, 


2OO  ZOOLOGY 

reproduction  takes  place  again,  the  embryos  migrate  into  the  muscles  and  the  new 
cycle  is  begun.  We  do  not  find  here  the  non-sexual  reproduction  that  helped  make 
the  Liver-fluke  so  prolific,  but  the  reproductive  power  of  Trichinella  is  very  great 
without  this.  It  is  estimated  that  an  ounce  of  "measly  "  pork  may  contain  80,000 
cysts  of  Trichinella,  and  that  each  female  produced  from  these  embryos  may  con- 
tain at  one  time  1,000  or  more  embryos.  During  her  life  she  may  produce  ten 
times  this  number.  Thus  the  40,000  females  from  such  a  meal  would  soon  supply 
40,000,000  young  worms  for  the  infection  of  the  muscles,  with  the  ability  of  renew- 
ing the  supply  at  short  periods.  Perfect  cooking  is  the  only  sure  safeguard  against 
the  possibility  of  infection. 

The  hookworm,  Necator  americanus,  belongs  to  this  phylum  and  is  a  common 
parasite  of  man  in  the  southeastern  states.  It  has  only  one  host  in  its  life  cycle. 
The  adult  worms  attach  to  the  wall  of  the  intestine  with  a  sucker-like  mouth. 
The  teeth  pierce  the  wall  and  the  esophagus  works  as  a  pump  to  extract  the  blood. 
From  the  mouth,  secretions  are  poured  that  prevent  the  blood  from  clotting. 
In  this  way  the  wound  may  continue  to  bleed  after  the  worm  goes  to  a  new  spot. 
One  worm  may  thus  make  many  wounds ;  and  in  some  cases  more  than  a  thousand 
have  been  found  in  one  person.  The  female  may  produce  thousands  of  eggs,  but 
these  cannot  develop  in  the  intestine  of  man.  They  pass  from  the  intestine,  and 
then  hatch  and  undergo  their  early  development  in  the  moist  soil.  If  they  do  not, 
at  a  certain  stage,  find  their  way  back  into  man  or  some  similar  host  they  die. 
Possibly  they  may  get  back  into  the  intestine  on  raw  vegetables,  but  a  much  more 
wonderful  way  has  been  demonstrated.  It  has  been  found  that  they  can  pene- 
trate the  skin,  and  many  of  the  poorer  people  go  barefoot  in  these  regions.  The 
larvae  bore  into  the  capillaries,  are  taken  to  the  heart  and  go  with  the  blood  to  the 
lungs.  Here  they  bore  into  the  lung  cavity,  pass  up  the  bronchial  tubes,  through  the 
trachea  into  the  gullet  and  on  into  the  intestine.  It  takes  about  seventy  days  from 
the  time  the  larvae  enter  the  skin  until  a  new  generation  of  eggs  appear  from  the 
intestine.  The  effect  of  infection  upon  human  beings  is  bloodlessness,  weakness, 
deranged  digestion  and  poor  nutrition,  abnormal  appetite,  and  laziness. 

It  has  been  found  that  the  worms  may  be  driven  from  the  intestine  by  thymol, 
which  stuns  the  worms  and  makes  them  loosen  their  hold,  followed  by  epsom  salts 
which  flushes  them  from  the  intestine.  Thymol,  however,  should  be  taken  only 
by  the  prescription  of  a  physician  as  it  acts  strongly  on  the  heart.  Prevention  of 
infection  involves  stopping  the  miscellaneous  infection  of  the  soil  by  discharges 
from  the  intestine  and  protecting  the  skin  from  exposure.  Drouth  and  freezing 
are  fatal  to  the  larvae. 

Phylum  VI:  Trochelminthes  (wheel-worms  or  rotifers'). — The  Rotifers  or  wheel- 
animalcules  are  microscopic  animals.  They  usually  tend  toward  bilateral  sym- 
metry. The  anterior  end  possesses  a  retractile  disc  supplied  with  cilia  variously 
arranged,  the  rhythmic  motions  of  which  often  give  the  appearance  of  a  rotating 
wheel.  From  this  the  name  of  the  group  comes.  This  organ  assists  in  locomotion 
and  produces  currents  in  the  water  by  which  food  is  brought  within  reach  of  the 
mouth.  There  is  a  digestive  tract  with  both  mouth  and  anus.  The  pharynx  into 
which  the  mouth  opens  is  provided  with  a  chitinous  grinding  apparatus  (mastax). 
Usually  a  pair  of  digestive  glands  open  into  the  stomach.  The  nervous  system  is 
usually  limited  to  a  single  ganglion  dorsal  to  the  pharynx.  Eye-spots  and  other 
sense  organs,  called  tactile  rods  or  antennae,  are  present.  There  is  a  true  ccelom 
communicating  with  the  exterior  by  means  of  excretory  tubules.  For  a  diagram- 
matic view  of  these  structures  see  Fig.  95. 


UNSEGMENTED    WORMS 


201 


The  sexes  are  distinct  and  are  frequently  very  different  in  appearance.  The 
males  are  often  much  smaller  than  the  females,  are  much  less  numerous,  and  are 
often  degenerate.  The  summer  eggs  are  of  two  kinds — large  and  small — and 
develop  parthenogenetically.  The  large  eggs  produce  females  and  the  small, 
males.  The  winter  eggs  have  a  thick  shell  and  are  believed  to  require  fertiliza- 
tion in  order  to  develop.  They  rest  during  the  winter  and  in  the  spring  develop 
into  females.  Development  is  direct.  The  adult  condition  in  the  Rotifers  sug- 
gests the  larval-  (trochophore)  condition  in  some  Annulata.  There  are  some  traces 

FIG.  95. 


----ft 


FIG.  95.  Diagram  of  a  sagittal  section  of  a  Rotifer.  Jb,  brain;  bl.,  excretory  bladder;  c,  cloaca; 
the  common  opening  of  digestive  and  reproductive  organs;  co,  coelom;  e,  eyespot;  ex,  excretory 
canal;  /,  flame  cells;  f.g.,  foot  gland;  ft.,  foot;  g,  gut;  m,  mouth;  m.f.,  longitudinal  muscle  fibres, 
mx,  mastax;  o,  ovary;  ph.,  pharynx;  s.g.,  salivary  gland;  t,  tentacle;  tr,  trochus,  or  cilia-bearing  disc. 

Questions  on  the  figure. — What  sets  of  organs  and  functions  are  indicated  in 
the  diagram?  Does  this  seem  a  lower  or  higher  form  than  the  other  types  studied 
in  this  chapter?  What  are  your  grounds  for  your  answer?  What  indications  of 
segmentation  are  represented  in  the  figure?  Is  the  mastax  in  the  stomodaeum  or 
mesenteron?  Where  do  the  various  authors  classify  Rotifers? 


2O2 


ZOOLOGY 


of  external  segmentation  in  the  tail  or  foot  region  in  some  species  and  for  these 
reasons  some  authors  class  the  Rotifers  near  the  Annulata.  Rotifers  are  aquatic, 
being  more  common  in  fresh  water  than  in  the  sea.  They  are  abundant  in  water- 
troughs,  gutters,  ponds.  They  are  capable  of  resuming  activity  after  having  been 
dried  up  in  the  mud  for  a  year  or  more.  This  power  must  be  of  great  value  in 
preserving  the  species  as  well  as  in  spreading  it. 

Phylum  VII: — Molluscoidea  (mollusk-like} . — The  two  groups  included  here 
are  quite  diverse  in  general  appearance  and  habit.  They  are  probably  not  as 
closely  related  as  this  classification  would  suggest.  Their  larval  stages  have  more 
points  in  common  than  the  adult.  There  is  in  the  adult  a  variously  shaped  ten- 
tacle-bearing ridge  (lophophore}  about  the  mouth.  The  central  nervous  system 
consists  of  one  or  two  ganglia  about  the  esophagus.  The  Brachyopoda  have  often 
been  grouped  with  the  mollusks,  but  authors  are  agreed  that  much  of  the  seeming 
resemblance  to  mollusks  is  superficial. 

FIG.  96. 


FIG.  96.  A  fresh-water  polyzoan,  Plumatella.  From  Parker  and  Haswell,  after  Allman.  a, 
anus;  fu.,  funiculus,  a  band  of  tissue  anchoring  the  intestine  to  the  body  wall;  g,  ganglion;  int., 
intestine;  m,  mouth;  o,  esophagus;  r,  reproductive  gland;  rt,  retractor  muscle;  si,  stomach;  stat, 
statoblast;  t,  tentacles. 

Questions  on  the  figure. — Is  this  an  individual  or  a  colony?  What  is  the 
function  of  the  retractor  muscles?  To  what  degree  are  the  polyps  capable  of 
contraction  as  shown  in  the  figure?  The  value  of  this  power?  What  are  the 
statoblasts? 

Class  I.  Polyzoa  (Bryozoa;  sea-mats;  corallines}. — The  Polyzoa  are  colonial 
animals  which  resemble  in  general  appearance  some  of  the  compound  hydroids. 
The  individual  animals  however  are  very  different  in  their  structure.  They  are 
found  both  in  salt  and  fresh  water.  In  Polyzoa  (Fig.  96)  the  digestive  tract  is 
sharply  bent,  the  anus  opening  close  to  the  mouth  either  within  or  outside  the 
circle  of  tentacles  (lophophore}.  A  distinct  coelom  is  typically  present.  There 
are  no  blood  vessels.  An  exoskeleton  is  formed  by  the  ectoderm,  by  means  of 
which  the  individuals  of  the  colony  are  held  together.  Each  member  of  the  colony 


UNSEGMENTED    WORMS  203 

may  retire  into  its  own  particular  portion  of  the  exoskeleton,  when  disturbed,  by 
the  contraction  of  appropriate  muscles.  The  brain  consists  of  a  single  ganglion 
lying  between  the  mouth  and  anus.  The  two  sexes  usually  occur  in  the  same 
individual.  The  colonies  are  formed  by  budding  which  takes  place  in  each  species 
in  a  way  that  is  characteristic  of  that  species.  Thus  it  comes  about  that  the  colo- 
nies differ  as  much  in  general  appearance  as  their  individuals  do  in  structure. 

Class  II.  BracTiiopoda  (arm-footed;  lamp-shells). — Brachiopods  are  marine 
forms  chiefly  of  geological  interest,  as  there  are  at  present  only  a  few  living  species. 
They  were  very  prevalent  in  early  geological  times.  They  possess  a  bivalved 
shell  which  suggests  that  of  the  bivalve  Mollusca  (as  the  clam).  From  this 
external  resemblance  they  have  long  been  classed  as  mollusks.  The  valves  are 
strictly  dorsal  and  ventral  in  the  Brachiopods,  however;  whereas  in  the  mollusks 
they  are  right  and  left.  Their  shell  is  therefore  no  longer  considered  as  homologous 
with  the  mollusk  shell.  The  internal  structure  is  still  further  removed  from  that 
of  the  clam.  The  digestive  tract  is  often  bent  much  as  in  the  Polyzoa,  and  the 
mouth  is  surrounded  by  a  tentacle-bearing  lophophore  (the  "arms").  The 
lophophore  may  have  a  skeletal  support  which  in  different  types  assumes  different 
shapes  (loop,  helix,  or  spiral).  A  peduncle  usually  extrudes  at  the  hinge,  by  means 
of  which  the  animal  attaches  itself  to  foreign  objects.  The  Brachiopods  are  not 
colonial.  The  student  is  referred  to  the  more  extended  texts  for  illustrations  of 
this  group  of  animals. 

236.  Some  Forms  of  Doubtful  Relationship. — The  old  group  "Vermes,"  or 
worms,  has  in  recent  years  had  several  fairly  definite  phyla  removed  from  it. 
Clearest  among  these  is  the  phylum  Annulata  or  segmented  worms  (Ch.  XV). 
More  recently  the  Phyla  Platyhelminthes,  Nemathelminthes,  Trochelminthes, 
and  Molluscoidea  have  been  recognized  by  many  students.  In  this  old  waste- 
basket  of  worm-like  animals  are  still  the  raw-materials  for  several  more  phyla 
when  we  learn  enough  about  them  to  distinguish  them.  None  of  these  groups 
is  numerous  in  species,  or  of  much  economic  or  human  interest.  They  cannot 
receive  much  attention  in  an  elementary  course.  Among  them  are: 

1.  Mesozoa,  which  are  simple,  parasitic  forms  less  complex  even  than  the  diplo- 
Uastic  animals.     It  ha?  been  thought  that  they  are  intermediate  between  protozoa 
and  metazoa. 

2.  Nemertinea,  similar  in  some  ways  to  flat  worms.     They  are  peculiar  in  de- 
velopment, and  have  some  systems  of  organs  better  developed  than  other  un- 
segmented  worms.     They  live  in  water,  chiefly  salt,  and  in  moist  earth.     Some 
attain  a  length  of  ninety  feet.    They  are  slender  and  usually  flattened. 

3.  Acanthocephala,  spine-headed  parasites  sometimes  classed  with  the  nema- 
todes.     Pound  in  most  vertebrates. 

4.  Chatognatha,   which   includes    Sagitta,    the   arrow- worm.    Free-swimming 
marine  animals,  sometimes  placed  near  the  segmented  worms. 

5.  Gephyrea,  marine  worms  living  in  sand  or  mud,  unsegmented  with  large 
body  cavity,  and  protrusible  proboscis.    The  sipunculids  are  the  best  known 
representatives. 

The  student  is  asked  to  seek  figures  of  some  of  these  rarer  types  in  the  larger 
texts,  and  notice  the  various  ways  in  which  they  are  classified. 

237.  Notes  on  Ecology  and  Distribution. — The  organisim 
included  in  this  chapter  represent  the  most  varied  modes  of  self. 


204  ZOOLOGY 

The  Turbellarians  are  free  animals  and  may  be  terrestrial, 
fresh- water  or  marine;  the  Rotifers  are  as  a  rule  free-swimming 
and  occur  chiefly  in  fresh  water;  the  Polyzoa  are  aquatic, 
attached,  colonial  forms  but  lead  for  the  most  part  an  independ- 
ent existence,  or  may  occasionally  be  commensal  with  other 
types  of  animals;  the  Brachiopods  are  marine  and  may  be 
attached,  but  are  not  colonial;  the  Trematodes  and  Cestodes 
represent  all  kinds  and  degrees  of  parasitism.  Even  if  all 
these  classes  of  animals  could  be  considered  akin,  their  habits 
of  life  and  their  consequent  adaptations  are  so  various  as  to 
produce  the  greatest  range  of  general  form  and  special  structure. 
If  we  consider  the  relatively  small  number  of  species  of 
animals  in  these  groups,  the  species  of  the  Platyhelminthes  are 
among  the  most  widely  distributed  of  the  metazoa.  This  is 
true  both  of  the  free  Turbellaria  and  Nematodes  and  the  parasitic 
Trematodes  and  Cestodes.  There  is  probably  not  a  large 
species  of  the  higher  Metazoa  which  escapes  being  the  host  of 
one  or  more  of  these  worms  at  some  stage  of  its  life  history. 
The  fact  of  parasitism,  the  ability  to  carry  on  the  life  cycle  in  a 
series  of  hosts,  and  the  prevalence  of  the  carnivorous  habit 
among  its  hosts  all  help  the  distribution.  The  organs  more 
commonly  infested  by  the  parasites  are  the  digestive  tube,  the 
blood  and  lymphatic  vessels,  the  ccelomic  cavity  or  other 
organs  where  the  nutritive  fluids  of  the  body  are  abundant. 
They  produce  all  sorts  of  disorders  from  mere  functional  dis- 
turbance (such  as  digestive  disorders  and  anaemia  from  the 
presence  of  the  tape-worm)  to  the  destruction  of  the  tissues  of 
the  organs  involved.  It  is  very  commonly  true  that  the  adult 
or  sexually  mature  individuals  are  produced  in  one  host,  and 
the  eggs  or  larvae  produced  by  them  find  their  way  into  another 
species  of  host  where  a  portion  of  the  development  toward 
maturity  occurs.  The  transfer  of  the  parasite  from  the  second 
back  to  the  first  host-species  is  necessary  to  complete  the  cycle. 
In  some  instances  there  is  not  a  change  from  one  animal  to 
another,  but  merely  from  one  organ  to  another  in  the  same 
animal,  as  in  T&nia  murina  of  the  rat.  In  size  the  unseg- 
mented  worms  vary  from  minute  microscopic  dimensions  to 
thirty  feet  in  length  in  the  tape-worm,  Bothriocephalus  latus. 


UNSEGMENTED    WORMS  205 

Some  suggestion  of  their  importance  to  man  and  the  higher 
animals  may  be  gathered  by  reference  to  the  following  table 
(p.  206). 

238.  Supplementary  Studies  for  the  Library. 

1.  In  what  different  ways  are  the  forms  included  in  this 
chapter  classified  in  the  various  textbooks  to  which  you  have 
access  ? 

2.  Consider  the  economic  importance  of  the  parasites  in- 
cluded in  this  chapter. 

3.  Make  a  further  study  of  the  life  histories  of  selected  repre- 
sentatives of  these  parasites. 

4.  Illustrate  by  means  of  the  unsegmented  worms  the  de- 
generation and  simplification  which  attends  parasitism. 

5.  In  what  various  ways  do  the  intestinal  parasites  in  the 
group  adhere  to  the  walls  of  the  digestive  tract  of  the  host  ? 

6.  Do  you  think  the  domestic  animals  are  more  or  less  likely 
to  be  attacked  and  suffer  from  these  internal  parasites  than  the 
wild  ?     What  evidences  would  you  offer  for  your  view  ? 

7.  Prepare  for  the  class  a  diagram  of  the  reproductive  organs 
in  the  Tape-worm,  indicating  the  function  of  each  of  the  por- 
tions. 

8.  What  is  meant  by  the  ' ' dermo-muscular "  sac  in  worms? 
Its  functions  ? 

9.  Report  on  the  importance  of  the  Brachiopods  in  early 
geological  time,  with  the  main  structural  features  of  the  class. 


206 


ZOOLOGY 


-SEXUAL  STAG 


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CHAPTER  XIV 

PHYLUM  VIII.— ECHINODERMATA   (STARFISH,   SEA-URCHINS,   SAND- 
DOLLARS,  SEA-LILIES) 

LABORATORY  EXERCISES 

239.  Asterias  (Starfish). — Both  dry  and  alcoholic,  or  other- 
wise preserved,  materials  should  be  at  hand. 

1.  General  form. 

Central  disc. 

Rays;  number,  form,  size,  etc.  Compare  several  in- 
dividuals. 

Oral  surface  (contains  mouth);  aboral  surface.  Note 
the  general  differences  between  these  surfaces  both 
in  the  arms  and  the  disc. 

The  axis  of  an  arm  is  known  as  a  radius;  the  space  be- 
tween is  inter  radial. 

Is  the  body  bilaterally  symmetrical  or  radially  sym- 
metrical ?  Give  the  reasons  for  your  conclusion. 

2.  External  anatomy. 
Oral  surface. 

Mouth:  position  and  surroundings. 

Ambulacral  groove:  position,  relation  to  the  mouth, 

extent. 
Ambulacral  feet:  how  arranged?     Is  the  foot  hollow 

or  solid?     Pull  off  one,  and  examine  with  lens  or 

low  power  of  the  microscope. 
Aboral  surface. 

Madreporic   body:   position    (radial   or   interradial?), 

shape,  size,  structure. 
Bivium;  trivium  (see  text,  §243). 
Examine  the  spines  on  both  surfaces  and  determine 

the   arrangement   and   shape   in   different   regions. 

How  are  they  fixed  to  the  body  ? 
Pedicellariae  (at  the  base  of  the  spines);  papulae  (soft 

bodies  among  the  spines).     Examine  with  lens. 
207 


208  ZOOLOGY 

Make  an  outline  drawing  of  each  surface,  filling  in  the  de- 
tails of  the  disc  and  one  arm  and  showing  the  points  above  de- 
termined. Sketch  one  of  each  of  the  various  classes  of  spines 
in  profile. 

3.  Organs  of  the  body  cavity. 

Using  alcoholic  or  other  moist  preparations,  cut  into  one  side  of  an  arm  of  the 
trivium,  making  an  incision  from  near  the  tip  almost  to  the  disc.  Cut  across  the 
back  of  the  arm  near  the  tip  and  make  a  similar  incision  on  the  other  side.  Lift 
the  flap  thus  separated  and  notice  the  organs  attached  to  it.  The  material  should 
be  dissected  under  water  or  50  per  cent,  alcohol,  or  kept  moistened  therewith. 
Hepatic  caeca;  extent,  number,  and  attachment. 

Detach  the  hepatic  caeca  from  the  aboral  wall  by  breaking  the  mesenteries, 
and  treat  all  the  arms  of  the  trivium  as  above. 

Carefully  connect  the  incisions  across  the  interradii  and  remove  the  entire 
aboral  wall  except  that  around  the  madreporic  body  and  that  between  it  and  the 
centre  of  the  disc,  being  careful  to  disturb  none  of  the  soft  parts.     If  material  is 
scarce  the  teacher  should  make  a  few  dissections  to  be  used  as  demonstrations. 
Notice: 

Body  cavity,  its  extent  and  contents. 

Stomach:  pyloric    (aboral)    portion;    shape,    position.     Are    the    hepatic 

caeca  connected  with  it?     Verify.     (The  stomach  opens  aborally  into  a 

small,  short  rectum  and  anus  usually  very  difficult  of  demonstration.) 

Rectal  diverticula?     number  and  position? 

Cardiac  (oral)  portion  of  stomach;  pouches,  number  and  form;  retractor 

muscles,  attached  to  the  floor  of  the  arms. 
Mouth;  peristome. 

Remove  the  hepatic  caeca  from  one  arm  and  find  the  genital  glands  which 
lie  in  the  floor  of  the  body  cavity.     What  is  their  number  and  arrange- 
ment?    At  what  point  do  they  connect  with  the  body  wall?     Can  you 
prove  that  they  communicate  with  the  exterior? 
Ampullae  (on  ventral  floor) :  determine  if  they  connect  with  ambulacral 

feet. 

Make  three  diagrams  showing  the  position  of  the  organs  thus  far  studied: 
(i)  the  aboral  surface  with  the  wall  removed,  showing  the  stomach  in 
the  disc,  the  hepatic  caeca  in  one  arm,  the  reproductive  bodies  in  a 
second,  and  the  ampullae  and  retractor  muscles  in  a  third;  (2)  a  trans- 
verse section  of  an  arm  about  midway  between  its  ends;  and  (3)  a  sagittal 
section  of  an  arm  continued  through  the  disc. 
4.  Ambulacral  system. 

In  a  specimen  from  which  the  preceding  organs  have  been  removed,  make  a 
transverse  section  of  an  arm  about  an  inch  from  the  disc.  Find  the  radial  water 
canal,  a  small  tube  lying  just  outside  the  skeleton  in  the  ambulacral  groove.  Force 
air  into  it  with  a  blow-pipe,  or  inject  a  colored  solution  with  a  hypodermic  syringe. 
What  other  structures  are  affected?  Trace  connection  between  radial  canal, 
ampulla,  and  ambulacral  feet.  Compare  the  number  of  ampullae  and  the  number 
of  feet.  Follow  the  radial  canal  toward  the  disc.  How  does  it  terminate? 

From  the  madreporic  body  trace  the  S-shaped  stone  canal  toward  the  oral 
surface.  How  does  it  terminate? 


ECHINODERMATA  2OQ 

Ring  canal:  its  position.  Are  there  any  other  structures  (sacs)  in  communi- 
cation with  the  circum-oral  ring-canal  beside  the  stone  canal  and  the  radial  water- 
tubes?  form  and  position? 

5.  Nervous  system. 

There  is  a  radial  nerve  (in  the  skin)  superficial  to  the  radial  water  canal  in  each 
arm.  The  radial  nerves  unite  to  form  a  circumoral  nerve  ring. 

6.  Skeletal  parts. 

Dried  material  and  portions  soaked  for  a  day  or  so  in  a  10  per  cent,  solution  of 
potash  should  be  used  to  supplement  the  alcoholic  specimens. 

Is  the  skeleton  complete,  i.e.,  are  the  ossicles  in  contact? 

Are  they  similarly  arranged  on  the  aboral  and  oral  surfaces?  Which  sur- 
face shows  the  greater  differentiation?  Illustrate,  and  find  a  reason  if  you  can. 
How  are  the  ossicles  related  to  the  spines?  to  the  papulae?  Study  with  some  care 
the  ossicles  forming  the  ambulacral  groove,  beginning  at  the  middle  line. 

Ambulacral  rafters:  shape  and  arrangement. 

Ambulacral  pores;  are  they  in,  or  between,  the  ossicles? 

Adambulacral  ossicles  (just  lateral  to  the  former);  how  do  these  compare  in 
number  with  the  ambulacral  ossicles? 

"Cross-shaped"  ossicles. 

Which  of  the  above  bear  spines?     what  kind? 

Place  some  of  the  cleaned  ossicles  in  dilute  hydrochloric  acid.  Result?  What 
is  the  significance  of  this  result? 

7.  Physiological  experiments  are  possible  only  near  the  seashore.     The  animals 
must  be  kept  in  sea  water,  and  studied  soon  after  being  collected.     When  possible, 
locomotion,  the  action  of  the  ambulacral  feet,  feeding,  and  sensitiveness  should  be 
studied.     Do  you  find  any  indications,  among  the  specimens  provided,  of  the  power 
to  renew  a  lost  arm?     With  care  and  perseverance,  at  the  proper  time  of  year,  the 
sexual  elements  may  be  collected  and  the  maturation,  fertilization,  and  cleavage  of 
the  ovum  illustrated  in  this  group.     Teachers  in  inland  schools  should  procure, 
whenever  possible,  slides  demonstrating  the  early  development  of  the  starfish  or 
sea-urchin. 

8.  Compare  briefly  the  external  features  of  other  "stars"  with  that  already 
studied. 

240.  Sea-urchin  (Echinus  or  Arbacia}. 

A  few  skeletons  of  sea-urchins  and  sand-dollars  will  be  of  great  value  in  enabling 
the  pupil  to  see  how  the  same  general  plan  of  structure  may  be  varied,  in  different 
organisms. 

1 .  Spines  (if  present) :  arrangement  and  method  of  attachment.     Are  they  of 
the  same  appearance  and  composition  as  the  skeleton?     Do  you  find  any  signs 
of  the  former  presence  of  ambulacral  feet?     If  so  what,  and  how  arranged?     Can 
they  all  have  the  same  function  as  in  the  starfish?     Proofs? 

2.  Ossicles;  Make  out  the  boundaries.     Compare  with  the  condition  in  the 
starfish.     What  are  the  special  advantages  gained  by  each  arrangement?     Can 
you  find  anything  corresponding  to  ambulacral  ossicles?     (Look  for  the  pores.) 
What  corresponds  to  the  ambulacral  groove  in  Asterias?     Identify  the  interambu- 
lacral  ossicles.    How  arranged  ?     What  is  radial  and  what  interradial  in  the  urchin  ? 
What  in  the  sea-urchin  would  correspond  to  the  oral  and  aboral  surfaces  in  the 
starfish?     Evidences?     Find  the  madreporic  body.     Make  a  plot  of  all  the  os- 
sicles in  this  region,  noting  the  differences.     Find  the  genital  pores. 

14 


21O  ZOOLOGY 

3.  "Aristotle's  lantern1'  (the  mouth  apparatus). 

Examine  the  structure  as  a  whole.     How  related  to  the  body?     Study  the 
parts  in  their  relation  to  each  other.     Number  and  method  of  action  ? 

DESCRIPTIVE  TEXT 

241.  The  Echinoderms  (spiny- skinned)  form  a  very  distinct 
group  of  animals,  which  in  the  adult  condition  at  least  show  a 
decided  radial  symmetry.     They  possess  a  more  or  less  extensive 
calcareous  exo-skeleton  with  outwardly  directed  spines.     The 
starfishes,    sea-urchins,    brittle    stars,    sea-lilies,    and    sea-cu- 
cumbers are  representatives.     They  are  marine  in  habit  and 
may  be  either  fixed  or  slow-moving.     They  agree  with  the 
Ccelenterates  in  having  radial  symmetry,  and  in  the  absence 
of  a  well-marked  brain  and  other  signs  of  cephalization.     There 
is  considerable  ground  for  believing  that  this  is  an  outcome  of 
their  sluggish  habit,  since  the  larval  free-swimming  condition  is 
bilaterally  symmetrical,  and  radial  symmetry  is  clearly  adapted 
to  a  passive  life.     It  is  difficult  to  determine  the  relationships 
of  the  Echinoderms;  yet  it  seems  probable  that  their  ancestors 
were  bilateral  forms.     Perhaps  they  should  be  considered  as 
connected  with  the  worms  rather  than  with  the  Ccelenterates. 

242.  General  Characters. 

1.  Larvae  are  bilaterally  symmetrical;  in  the  adult  there  is 
a  more  or  less  complete  radial  arrangement  of  equivalent  parts, 
usually  on  the  plan  of  five.     In  this  radial  plan  all  the  princi- 
pal sets  of  organs  share:  as  the  nervous,  digestive,  reproduc- 
tive, etc. 

2.  There  is  a  complete  differentiation  of  digestive  tract  and 
body  cavity.     The  latter  is  large. 

3.  The    blood- vascular    system    is    partially    differentiated 
from  the  body  cavity,  but  communicates  with  it. 

4.  A  calcareous  exo-skeleton  occurs,  derived  from  the  meso- 
derm.     It  may  consist  of  isolated  spicules  or  united  plates. 
Associated  with  these  are  usually  spines,  from  which  the  group 
is  named. 

5.  A  water- vascular  system, — consisting  of  a  series  of  tubes 
(closed  except  at  one  point),  muscular  sacs  (ampullae)  and  dis- 
tensible feet, — serves  a  locomotor  and  respiratory  function. 


ECHINODERMATA 


211 


6.  Reproduction  sexual;  development  usually  indirect,  *.>., 
with  a  metamorphosis.  Reproduction  by  budding  does  not 
occur. 

243.  General  Survey. — The  majority  of  echinoderms  have 
a  central  disc  in  which  is  located  portions  of  the  various  sets 
of  organs.  Ordinarily  there  radiate  from  this  disc  more  or  less 
clearly  defined  rays  or  arms  in  which  lie  radial  outgrowths  from 

FIG.  97. 


FIG.  97.     Starfish,  from  chart  of  Leuckart  and  Nitsche. 

Questions  on  the  figure. — How  would  you  describe  the  symmetry  of  the  animal  ? 
Identify  and  name,  by  comparison  with  the  diagrams  and  the  text,  all  the  struc- 
tures which  show  in  the  figure.  Compare  this  with  specimens  or  figures  of  the 
common  American  species  and  note  the  chief  differences. 

certain  central  organs.  The  spaces  between  the  rays  (inter- 
radii)  may  be  bridged  by  growth  in  such  a  way  that  the  distinc- 
tion between  rays  and  disc  is  not  marked  (echinoids) .  In  crinoids 
the  arms  may  be  much  branched.  The  oral-aboral  axis  is  usu- 
ally pronounced,  often  short,  and  is  vertical  in  position  (aster- 


212  ZOOLOGY 

oids,  echinoids,  crinoids,  etc.),  though  in  the  sea-cucumbers 
(holothuroids)  it  is  horizontal  and  much  elongated.  Starfish 
are  flattened  vertically,  as  are  the  sand-dollars,  but  many  of  the 
urchins  (echinoids)  are  dome-shaped.  The  antimeres  are  at 
right  angles  to  this  chief  axis.  In  addition  to  this  dominant 
radial  symmetry,  there  is  seen  even  in  the  adult  a  suggestion 
of  the  bilateral  condition.  The  madreporic  body  generally 
occurs  in  only  one  interradius,  and  a  plane  passing  through 
it  and  splitting  the  opposite  arm  divides  the  body  into  two  sym- 
metrical halves.  No  other  plane  does  this.  The  two  arms 
embracing  the  madreporic  body  are  known  as  the  bivium,  the 
remaining  three,  the  trivium.  In  some  of  the  echinoids  the 
bilateral  symmetry  becomes  much  more  pronounced  than  in 
starfish. 

244.  The  integument  consists  of  an  outer  ectodermal  por- 
tion which  is  often  ciliated  (cilia  wanting  in  the  holothuroids 
and  ophiuroids),  and  a  subepithelial,  mesodermic  layer  in  which 
is  developed  the  calcareous  ossicles.     These  may  occur  as  spic- 
ules,  as  rods,  or  as  plates  in  the  various  classes.     They  may  be 
bound  together  by  connective  and  muscle  fibres.     Frequently 
the  ossicles  bear  spines  which  may '  or  may  not  be  movable. 
The  spines  are  useful  in  defense  and  locomotion.     Special  forms 
of   spines  known   as   pedicellaria  often   occur   (asteroids   and 
echinoids).     They    consist   of   two-    or   three-pronged   pincers 
moved  by  muscles.     They  may  be  mounted  on  short  stalks.     It 
is  suggested  that  they  help  clear  the  body  of  foreign  objects 
which  lodge  among  the  spines. 

245.  Digestive  System. — The  mouth  and  anus  usually  open 
at  opposite  poles  of  the  principal  axis  (asteroids,  holothuroids, 
and  some  echinoids).     When  the  axis  is  vertical  the  mouth  is 
usually  directed  downward,  in  the  centre  of  the  oral  surface, 
and  the  anus  occupies  a  more  or  less  central  position  on  the 
upper  or  aboral  surface.     In  some  of  the  echinoids  and  crinoids 
the  mouth  or  anus,  or  both,  have  vacated  their  central  position 
and  may  come  to  occupy  opposite  margins  of  the  body.     The 
digestive  tract  is  a  simple  tube,  in  the  holothuroids  running 
spirally  through  the  body.     In  the  echinoids  a  similar  condition 


ECHINODERMATA  2 1 3 

is  found  except  that  it  begins  in  a  complex  masticating  apparatus 
of  five  parts  (Aristotle's  lantern).  In  the  asteroids  the  mouth 
opens  by  a  short  esophagus  into  an  expanded  stomach  which  is 
divided  into  an  oral,  or  cardiac,  and  a  pyloric  portion  (Fig.  98). 
From  the  pyloric  part  the  narrow  intestine  passes  to  the  anus. 
Outpocketings  (caeca)  may  occur  in  any  of  these  divisions. 
The  most  important  are  the  hepatic  caeca  which  are  glandular  in 
function. 

FIG.  98. 


FIG.  98.  Vertical  (sagittal)  section  through  an  arm  and  an  interradius  of  a  Starfish  (diagram- 
matic), c,  anus;  amp.,  ampulla;  c.b..  circular  blood  vessel;  c.w.,  circular  water  canal;  co.,  ccelom; 
CO.  e.  ccelomic  epithelium;  d.b.,  dermal  branchiae;  e,  position  of  the  eyespot;  ect.,  ectoderm;  ent.t 
entoderm;  /,  ambulacral  foot;  g,  ambulacral  groove;  h,  hepatic  caeca  or  liver;  *',  intestine;  i.e., 
intestinal  caeca;  mes,  mesoderm;  mo.,  mouth;  m.p.,  madreporic  body;  n.r.,  nerve  ring;  os.,  ossicles 
in  mesoderm;  r.n.,  radial  nerve  band;  r.b.,  radial  blood  vessel;  r.p.,  reproductive  pore;  r.w.,  radial 
water  canal;  s.c.,  stone  canal;  sp.,  spines;  z,  lacunar  spaces  in  the  mesoderm.  (Adapted  from 
various  sources.) 

246.  The  body  cavity  is  well  developed  in  the  disc  and  usu- 
ally in  the  arms,  is  lined  with  a  ciliated  epithelium,  and  contains 
a  fluid  with  amoeboid  corpuscles.     It  is  completely  distinct  from 
the  digestive  cavity.     Thin  outgrowths  of  the  body- wall  (papula 
or  branchid)  contain  extensions  of  the  ccelom.     These  assist  in 
respiration. 

247.  Ambulacral  or  Water-vascular  System. — This  system 
of  tubular  organs  is  peculiar  to  the  echinoderms.     It  originates 
(see  also  254),  in  common  with  the  body  cavity,  as  an  outgrowth 
from  the  archenteron  and  is  to  be  regarded  as  a  specialized  por- 


214 


ZOOLOGY 


tion  of  the  body  cavity.  In  some  cases  these  two  cavities  are 
in  communication  in  the  adult.  It  consists  essentially  of  a 
ring-vessel  about  the  mouth  from  which  pass  radial  tubes,  one  in 
each  arm.  From  the  radial  tubes  arise  lateral  channels  which 
communicate  directly  or  through  bladder-like  ampullae,  with 
distensible  feet  which  reach  the  exterior  by  pores  in  the  skeleton 
(Figs.  99,  100).  The  tip  of  the  foot  may  be  provided  with  a 
sucking-disc,  serving  as  a  means  of  attachment  and  of  locomo- 
tion. Frequently  the  walls  of  these  feet  are  thin  and  apparently 
serve  for  respiration,  and  the  terminal  "foot"  at  the  end  of  each 


FIG.  99. 


d.  b. 


r.p™ 


FIG.  99.  Transverse  section  of  the  arm  of  a  Starfish  near  the  disc.  Diagrammatic  Lettering 
as  in  preceding  ^figure.  a,r.t  ambulacral  rafter  (ossicle);  ov.,  ovary,  containing  ova. 

|  Questions  on  figures  98  and  99. — What  are  the  principal  sets  of  organs  repre- 
sented^ the  disc  of  the  starfish  ?  Which  of  these  have  radial  portions  going  into  the 
arms?  Follow  carefully  the  ectodermal,  entodermal  and  mesodermal  boundaries. 
Locate  and  identify  the  various  structures  lettered,  and  determine  as  far  as  possible, 
whether  the  essential  part  of  each  is  furnished  by  ectoderm,  entoderm  or  mesoderm. 
Is  there  a  ccelom?  Your  evidences?  What  is  the  relation  of  the  water-vascular 
cavity  to  the  ccelom,  in  origin? 

radius  may  be  highly  modified  to  form  a  sense  organ  (tentacle.) 
The  feet,  the  ampullae,  and  even  the  radial  vessels  may  be  want- 
ing. The  ring-canal,  in  typical  forms,  communicates  with  the 
surrounding  sea- water  by  means  of  a  tube  (stone  canal)  which 
terminates  in  a  sieve-like  plate,  the  madreporic  body,  through 
which  the  water  enters  the  water- vascular  system.  In  the 


ECHINODERMATA  215 

majority  of  the  Holothuroids  the  madreporic  tubes  open  into 
the  body  cavity  instead  of  opening  to  the  exterior.  In  conse- 
quence the  fluid  which  is  found  in  the  water-vascular  system  is 
the  same  as  that  of  the  body  cavity  and  contains  amoeboid  cells. 
In  the  crinoids  also  the  water-vascular  system  communicates 
directly  with  the  ccelom,  but  there  is  no  true  madreporic  canal. 
In  its  stead  is  found  a  system  of  ciliated  water-tubes  in  connec- 
tion with  the  ring  canal.  Identify  the  elements  in  the  water- 
vascular  system  from  Fig.  100. 

248.  Respiration    occurs    in    connection    with    the    water- 
vascular  system  especially  in  those  forms  in  which  the  tentacles 
and  ambulacral  feet  are  possessed  of  thin  walls  (holothuroids 
and  some  echinoids).     In  the  asteroids  and  echinoids  there  are 
thin  outpocketings  of  the  body-wall,  papulae  or  branchiae  (Fig. 
99,  d.b.),  the  cavity  in  which  is  continuous  with  the  body  cavity. 
The  body  fluids  may  thus  be  aerated  from  the  water  outside. 
In  some  forms  water  is  drawn  into  special  branching  pockets 
(respiratory  tree)  in  the  wall  of  the  rectum,  and  later  is  forced 
out  again. 

249.  Circulation. — The  circulatory  vessels  are  merely  partly 
differentiated  portions  of  the  ccelom  or  body  cavity.     Our  knowl- 
edge is  by  no  means  complete  but  it  seems  that  in  none  of  the 
groups  is  there  a  complete  separation  of  the  blood  spaces  from 
the  ccelom.     There  are  probably  no  contractile  hearts.     The 
walls  of  the  blood  spaces  may  bear  cilia  which  assist  in  securing 
the  motion  of  the  fluid.     The  blood  contains  migratory  cells, 
usually  colorless,  and  is  identical  with  the  fluid  in  the  body 
cavity.     The  general  body  contractions  are  important  in  caus- 
ing motion  of  the  fluids.     It  should  be  remembered  that  the 
water- vascular  system  is  also  partly  circulatory  in  function. 
The  blood  vessels  of  the  various  classes  agree  in  having  a  central 
circular  portion  consisting  of  one  or  more  rings,  with  radial 
tubes  running  into  the  arms,  and  in  some  instances  vessels  which 
accompany  the  intestine.     The  vessels  of  the  oral  surface  are, 
throughout,  in  close  connection  with  the  nervous  epithelium 
(Fig.  98,  r.b.). 


2l6 


ZOOLOGY 


250.  Excretion. — It  is  impossible  to  name  any  organs  known 
to  be  solely  excretory  in  function.  As  in  respiration  many 
organs  may  take  part  in  the  work.  The  gaseous  and  soluble 
excreta  are  eliminated  through  the  general  body  surface,  the 
papulae,  the  respiratory  tree,  or  the  ambulacral  organs.  The 
skeletal  ossicles  in  the  mesoderm  represent,  in  part  at  least, 

FIG.  100. 


-rt. 


-f 


FIG.  100.  Diagram  of  a  portion  of  the  water- vascular  (ambulacral)  system  of  the  Starfish,  a 
ampullse;/,  ambulacral  feet;  m,  madreporic  body;  p,  Polian  vesicles;  r.r.,  ring  canal,  with  the  upper 
portion  removed  at  the  right  of  the  figure;  r.t.,  radial  water  tubes  (in  r.t.'  the  upper  portion  is  re- 
moved at  the  distal  end  and  the  proximal  portion  is  represented  entire) ;  s,  stone  canal. 

Questions  on  the  figure. — Where  does  the  water  enter  this  system  of  vessels  ? 
At  what  points  in  the  system  is  it  of  use  ?  By  comparing  all  illustrations  at  your 
disposal,  describe  the  mode  of  using  this  system  of  organs  for  locomotion.  How 
may  it  be  used  in  respiration? 

the  elimination  of  certain  inorganic  salts  which  can  not  be  used 
in  the  vital  activities  and  are  therefore  excretions. 

251.  The  Muscular  System. — The  degree  of  the  develop- 
ment of  the  muscular  system  varies.  In  those  forms  which 
have  a  well-developed  skeleton  the  body  muscles  are  not  of 


ECHINODERMATA  217 

much  significance.  In  the  holothurians,  on  the  contrary,  the 
body  is  capable  of  definite  and  considerable  contractions,  by 
reason  of  both  circular  and  longitudinal  fibres.  In  forms  with 
incomplete  skeletons,  as  the  starfish,  muscular  fibres  connect 
the  ossicles.  There  is  a  high  degree  of  flexion  of  the  arms  of 
crinoids  and  aphiuroids.  There  are  also  special  muscles  con- 
trolling the  water  vascular  system,  the  stomach,  the  mouth  parts, 
the  spines  and  pedicellariae.  The  fibres  are  non-striate. 

252.  The  Nervous  System  consists  of  a  ring  around  the 
mouth  and  a  radial  nervous  band  in  each  arm  supplying,  by  a 
plexus  of  fibres  and  cells,  all  the  radial  organs.     This  system  is 
superficial  (" ventral")  to  the  radial  water-tube  (Fig.  99,  r.  n.) 
and  in  the  starfish  preserves  its  connection  with  the  ectoderm 
from  which  it  is  in  all  forms  derived.     Other  deeper  lying,  and 
even  aboral,  nervous  elements  are  described  for  some  of  the 
members  of  the  group.     These  elements,  when  present,  have  as 
their  function  the  innervation  of  the  muscles  of  the  interior  and 
of  the  aboral  wall  of  the  body. 

Sensory  organs  are  not  highly  developed.  The  animals 
show  evidences  of  possessing  a  chemical  sense  (analogous  to 
taste  and  smell)  by  which  the  presence  of  food  is  detected. 
This  is  apparently  localized  in  the  tentacles  in  such  forms  as 
have  them.  A  tactile  sense  is  also  present,  and  is  most  highly 
developed  in  the  tentacles,  ambulacral  feet,  and  other  movable 
outgrowths.  At  the  tip  of  the  antimeres  of  the  asteroids,  or  of 
the  radial  nerve  (echinoid)  are  structures  bearing  pigmented 
spots,  which  appear  to  be  sensitive  to  light  (eye-spots).  These 
cannot  give  more  than  a  very  general  impression  of  light,  by 
means  of  the  chemical  changes  induced  in  the  pigment  cells  by 
the  action  of  the  light.  Starfish  are  more  quiet  during  the  day. 
When  the  starfish  is  placed  on  its  "back"  it  makes  definite 
coordinated  movements  to  right  itself.  Experiments  show  that 
the  starfish  is  modified  slowly  by  its  experiences.  That  is  to 
say  it  learns  to  do  certain  things  in  certain  ways.  It  retains 
("remembers")  for  at  least  a  few  days. 

253.  Reproduction  is  wholly  sexual.     The  sexes  are  distinct, 
but  the  males  and  females  are  not  often  distinguishable  by 


2l8  ZOOLOGY 

external  characters.  The  sexual  organs,  ovaries  or  testes,  are 
lobed  bodies  occurring  usually  in  pairs  in  an  interradial  position. 
These  open  by  pores  also  interradial,  and  usually  dorsal  (Fig. 
99,  r.  p.).  There  are  typically  five  pairs  of  genital  glands,  but 
in  the  holothurians  the  number  is  reduced  to  one.  Fertilization 
takes  place  outside  the  body,  and  usually  the  development  is 
wholly  independent  of  the  parent.  In  some  instances  however 
the  parent  has  special  pouches  in  which  development  proceeds. 

254.  Development. — The  fertilized  ovum  undergoes  total 
and  practically  equal  segmentation,  producing  a  ciliated  blastula. 
Gastrulation  occurs  by  invagination  resulting  in  ectoderm  and 
entoderm.  The  mesoderm  is  formed  in  two  ways:  (i)  by  mi- 
grating cells  budded  from  the  entoderm  into  the  segmentation 
cavity  (mesenchyma;  Fig.  14,  c}\  and  (2)  by  the  outgrowth  of 
ccelomic  vesicles  or  pouches  from  the  wall  of  the  archenteron 
or  entoderm  (true  mesoderm).  These  latter  outpockets  of  the 
wall  of  the  gut  are  those  which  give  rise  to  the  ccelom  and  to 
the  water  vascular  system  (see  §247). 

In  the  later  larval  development  the  cilia  of  the  gastrula  be- 
come limited  to  two  zones, — a  preoral  and  a  preanal, — and  the 
shape  of  the  larva  is  much  modified.  Numerous  paired,  lateral 
outgrowths  serve  to  accentuate  the  fundamental  bilateral  sym- 
metry. In  most  members  of  the  group  a  marked  metamorphosis 
occurs  in  the  passage  from  the  larval  to  the  adult  condition. 
During  this  change,  the  water  vascular  system  and  the  mid-gut 
of  the  larva  are  retained  with  the  necessary  modifications. 
About  these  as  a  centre,  what  we  might  almost  call  a  new  animal, 
the  radiate  starfish,  begins  to  grow  at  the  expense  of  the  larval 
organs  which  are  absorbed  by  the  amoeboid  cells,  and  thus  new 
organs  appropriate  to  the  adult  are  formed.  During  this  process 
the  bilateral  symmetry  of  the  embryo  gives  place  to  the  radial 
symmetry  of  the  adult.  While  there  is  no  reproduction  by 
budding  there  is  a  striking  power  of  renewal  of  arms  or  other 
portions  which  may  be  lost  by  injury,  or  in  some  instances  by 
self -mutilation.  Arms  are  readily  reproduced  if  the  disc  is 
uninjured  (stars,  brittle-stars,  and  crinoids);  portions  of  the 
internal  organs,  as  the  digestive  tract,  are  said  to  be  regenerated 


ECHINODERMATA  2 19 

by  some  of  the  holothurians.  Occasionally,  at  least,  an  arm 
and  a  small  portion  of  disc  seems  to  have  the  power  of  reproduc- 
ing a  new  disc  and  other  arms.  This  power  of  throwing  off  arms 
and  replacing  them  is  doubtless  a  means  of  protection. 

255.  Ecology. — The  echinoderms  are  marine.  The  larvae 
are  free-swimming, — pelagic, — but  after  the  assumption  of  the 
adult  form  they  usually  become  much  less  active.  The  crinoids 
are  typically  stalked  and  often  attached.  The  asteroids  and 
echinoids  inhabit  the  bottom  of  the  ocean  where  they  creep  more 
or  less  slowly.  They  may  be  found  at  almost  any  depth,  from 
the  shallow  pools  at  low  tide  to  the  deepest  bottoms.  Many 
of  them  burrow  in  the  mud  and  sand,  and  others  (some  sea- 
urchins)  have  the  power  of  scouring  out  burrows  in  the  rocks 
by  the  action  of  their  spines.  Echinoderms,  being  slow  movers, 
are  compelled  to  subsist  upon  such  food  as  the  currents  or  the 
chance  movements  of  other  animals  may  bring,  or  upon  the 
debris  which  falls  to  the  bottom  of  the  sea,  or  upon  such  organ- 
isms as  are  attached  and  cannot  escape.  The  starfishes  for 
example  are  a  constant  menace  to  the  oyster  beds.  The  fact 
that  some  starfish  are  in  a  measure  gregarious  makes  this  all 
the  more  serious.  It  is  difficult  to  see  how  the  starfish  can  get 
the  oyster  from  the  protection  of  its  shell,  but  it  manages  to  get 
the  shell  open  and  clasping  its  arms  about  its  prey  it  turns  the 
cardiac  portion  of  its  stomach  inside-out  over  the  soft  part  of 
the  oyster  and  thus  leisurely  digests  it  outside  its  body,  so  to 
speak,  leaving  the  empty  shell  behind.  Except  for  this  the 
group  is  of  little  economic  importance.  The  Chinese  esteem 
some  species  of  Holothuria  (the  trepang,  for  example)  as  food. 
The  group  appeared  early  in  geological  time  and  has  had  very 
characteristic  representatives  in  all  ages  up  to  the  present. 
The  changes  which  have  taken  place  in  the  echinoderms  from 
one  geological  age  to  another  are  among  the  most  interesting  and 
instructive  furnished  by  the  invertebrates. 

256.  Classification. — Class  I.,  Blastoidea;  Class  II.,  Cystoidea. 

(These  are  both  extinct,  fossil  classes.  They  comprise  stalked  and  attached 
forms,  and  perhaps  represent  the  nearest  approach  of  our  known  species  to  the 
primitive  echinoderms.) 

Class  III.     Crinoidea  (feather-stars  and  sea-lilies}. — These  forms  are  less  com- 


22O  ZOOLOGY 

mon  than  in  earlier  geological  times,  when  they  must  have  been  very  abundant 
and  very  beautiful.  They  contribute  much  to  the  formation  of  the  limestone  of 
the  Palaeozoic.  They  are  usually  provided  with  jointed  stalks,  by  which  they  may 
be  attached  to  the  bottom.  At  the  summit  of  the  stalk  is  a  central  disc  with  five 
arms  often  much  branched  and  bearing  lateral  pinnules.  The  anus  is  on  the  oral 
or  upper  surface,  the  stalk  arising  from  aboral  surface.  They  are  inhabitants  of 
moderate  to  deep  seas. 

Class  IV.  Asteroidea  (starfishes)  (Fig.  97). — The  asteroids,  of  which  there 
are  several  hundred  species,  are  free  echinoderms  with  a  central  disc  and  usually 
five  arms.  The  latter  are  large  and  contain  liberal  ccelomic  spaces  in  which  are 
lodged  outgrowths  of  the  digestive  system  and  other  organs.  There  is  a  distinct 
oral  and  aboral  surface.  The  anus  and  madreporic  body  are  on  the  latter.  Dis- 
tinct ambulacral  grooves  lie  on  the  oral  surface  of  the  arms.  Adult  starfish  may 
vary  in  size  from  a  few  inches  to  two  feet  or  more  in  diameter. 

Class  V.  Ophiuroidea  (serpent-stars). — These  are  fragile,  free  echinoderms 
in  which  the  arms  are  small  and  much  more  distinct  from  the  disc  than  in  the 
asteroids.  The  organs  of  the  disc  are  not  all  continued  into  the  arms.  There  is 
no  anus,  no  ambulacral  grooves,  and  the  madreporic  body  is  on  the  oral  surface. 
Their  slender  arms  are  useful  in  clinging  to  supports  or  to  prey,  and  are  used  in 
locomotion.  They  are  readily  broken  and  regenerated. 

Class  VI.  Echinoidea  (sea-urchins,  sand-dollars). — These  are  free  echinoderms 
without  free  arms.  The  arms  are  connected  by  the  development  of  interradial 
plates.  The  calcareous  rods  are  united  into  plates  which  produce  a  complete  exter- 
nal skeleton  varying  from  flat  dome-shape  (as  in  sand-dollars)  to  a  globular  form 
(Echinus  or  Arbacia).  The  mouth  is  usually  in  the  centre  of  the  oral  surface  and 
the  anus  near  the  centre  of  the  aboral,  yet  one  or  both  may  come  to  have  an  excen- 
tric  position.  In  this  way  the  bilateral  symmetry  is  accentuated  at  the  expense  of 
the  underlying  radial  symmetry.  The  madreporic  body  is  aboral  and  there  are 
no  ambulacral  grooves.  The  spines  of  the  urchins  are  usually  well  developed 
and  may  be  used  to  scour  out  rounded  pockets  in  rock  in  which  the  animals  are 
sometimes  found. 

Class  VII.  Holothuroidea  (sea-cucumbers}. — These  are  soit,  free  echinoderms, 
elongated,  cylindrical  or  flat,  with  mouth  and  anus  at  opposite  poles  of  the  hori- 
zontal long  axis.  The  skeleton  is  not  well-developed,  usually  being  represented 
merely  by  scattered  spicules.  The  water- vascular  system  in  most  forms  com- 
municates with  the  body  cavity  instead  of  the  exterior.  Well-developed  tentacles 
occur  about  the  mouth.  Most  holothurians  burrow  in  the  sand  or  mud,  but  others 
cling  to  rocks  near  the  surface  of  the  water,  and  still  others  occur  at  great  depths 
in  the  ocean.  Their  reactions  are  rather  more  complex  than  in  the  other  members 
of  the  phyla.  They  are  more  muscular  and  more  responsive  to  light  and  contacts. 
In  addition  to  hiding  by  burrowing,  they  often  contract  the  muscular  wall  violently 
and  eject  large  portions  of  their  viscera.  These  may  be  replaced  by  regeneration. 

257.  Suggestive  Studies  for  the  Library  or  Laboratory. 

1.  Read  and  report  on  the  metamorphosis  of  the  various 
members  of  the  group. 

2.  Study  from  dry  and  moist  material  and  report  on  the 
structure  and  mode  of  action  of  "Aristotle's  lantern"  in  Echinus. 


ECHINODERMATA  221 

3.  Construct  a  table  of  parallel  columns — one  for  each  of 
the  five  living  classes — and  contrast  them  as  to:  (i)  general 
form  of  body  including  symmetry,  (2)  manner  of  motion,  (3) 
position  of  mouth  and  anus,  (4)  position  of  madreporic  body,  (5) 
character  of  digestive  tract,  (6)  differences  in  the  spines  and  other 
skeletal  structures,  (7)  the  position  and  character  of  the  ambu- 
lacral  feet,  (8)  habitat  and  food,  (9)  parts  repeated  in  the  anti- 
meres. 

4.  Report  on  the  habits,   appearance,   and  abundance  of 
crinoids  in  geological  time. 

5 .  The  origin  and  development  of  the  water- vascular  system. 

6.  Compare  the  figures  of  the  various  classes  as  illustrated 
in  your  reference  texts  and  mark  the  degree  of  variation. 


CHAPTER  XV 
PHYLUM  IX.— ANNULATA  (SEGMENTED  WORMS) 

LABORATORY  EXERCISES 

258.  The  Earthworm  (Allolobophora  or  Lumbricus). — The 
principal  work  should  be  done  with  living  worms.  For  what- 
ever anatomical  work  is  undertaken,  specimens  may  be  killed 
by  exposure  to  fumes  of  chloroform  while  wrapped  in  cloth 
moistened  with  water;  they  should  then  be  pinned  out  straight, 
and  hardened  in  an  abundance  of  alcohol.  If  needed  in  the 
winter  they  may  often  be  found  under  manure  heaps,  or  about 
green-houses.  They  may  be  kept  alive  in  flower  pots  contain- 
ing moist  earth. 

1.  Promorphology;  General  Form. — Is  there  an  anterior  and 
a  posterior  end  ?     How  distinguished  ?     Is  there  any  distinction 
of  dorsal  and  ventral  surfaces  ?     If  so,  what  ?     Is  there  bilateral 
symmetry  ?     What  external  evidences  of  segmentation  do  you 
find?     How  are  the  similar  units  (metameres  or  segments)  ar- 
ranged?    Compare  with  the  condition  in  the  starfish.     Com- 
pare the  metameres  of  different  parts  of  the  body,  noting  differ- 
ences.    Is  the  body  divisible  into  regions  (i.e.,  groups  of  similar 
metameres)  ?     Locate   (by  numbering  the  segments)   all  such 
regions.     How  many  segments  in  the  animal  ?     To  what  extent 
does  this  vary  in  different  specimens?     Show  by  a  series  of 
diagrams  the  shape  of  the  animal,  and  the  shape  and  size  of 
cross  sections  in  various  regions. 

2 .  Activities. — Describe,  after  careful  observation,  the  method 
of    locomotion    in    the    earthworm.     Place    the    worm    on    a 
rough  board ;  on  a  plate  of  glass.     What  is  the  difference  ?     And 
why  ?     Compare  the  various  parts  of  the  body  as  to  size,  during 
movement.     Cause  of  the  difference  ?     Can  each  end  move  fore- 
most?    What  seems  to  determine  which  end  shall  protrude  as 
the  result  of  the  muscular  contractions  ? 

222 


ANNULATA 


223 


Does  the  animal  respond  equally  to  contact  (with  pencil  or 
toothpick)  at  anterior,  posterior,  and  middle  parts  of  the  body  ? 
Devise  a  method  of  determining  whether  it  is  sensitive  to  light. 
Record  results. 

Place  moist  soil  and  dry  soil  side  by  side  on  a  board;  place 
the  worm  in  various  positions  to  test  his  preference.  Record 
results.  Place  a  piece  of  filter  paper  which  has  been  dipped  in 
acetic  acid  in  the  path  of  a  worm.  How  does  it  react?  Try 
similarly  a  sugar  solution;  a  salt  solution;  a  decoction  of  decay- 
ing leaves.  Will  an  earthworm  pass  into  water  ?  Do  your  ex- 
periments bear  in  any  way  on  the  habits  of  the  earthworm  in 
nature?  Can  you  secure  any  evidence  as  to  the  food  of  the 
earthworm  ?  How  ?  Record  your  results. 

3.  Special  External  Structures. — Locate  the  mouth,  the  pre- 
oral  lobe,  clitellum  (a  series  of  swollen  segments),  anus.  Com- 
pare preoral  lobe  with  other  segments.  With  a  lens  and  by 
drawing  the  worm  backward  between  the  fingers  discover  the 
setae  or  bristles.  Are  they  found  on  all  segments?  Number 
and  position  of  the  groups  of  setae  in  each  segment  ?  What  is 
the  function  of  the  setae  ?  Proofs  ? 

4.  Internal  Anatomy. — Pin  out  a  large  specimen,  which  has  been  hardened  in 
alcohol,  on  dissecting  board  or  pan,  and  carefully  remove  the  dorsal  wall  from  the 
anterior  half  of  the  body  by  making  lateral  incisions  with  sharp-pointed  scissors, 
or  make  a  single  incision  along  the  back  a  little  to  one  side  of  the  middle  line. 
After  noting  the  cross  membranes  (dissepiments},  their  relation  to  the  rings  on  the 
outside,  and  their  attachments,  cut  them  so  the  body  wall  may  be  folded  back  and 
pinned.  The  dissection  should  proceed  under  fluid, — 50  per  cent,  alcohol,  for 
example.  Make  all  the  outline  drawings  necessary  to  show  all  your  discoveries. 
Notice  the  coelom.  It  is  completely  divided  by  the  dissepiments?  Are  the  cham- 
bers of  equal  size? 

(a)  Digestive  organs:  Beginning  at  the  anterior  end,  note  the  following  re- 
gions: 

Pharynx,  a  pear-shaped  enlargement:  how  held  in  place?  In  what  seg- 
ments is  it  situated  ? 

Esophagus,  a  narrow  tube;  crop;  gizzard;  intestine. 

Determine  the  segments  in  which  each  region  occurs.  Does  the  digestive 
tract  show  any  signs  of  segmentation,  i.e.,  in  correspondence  with  the 
external  rings? 

(6)  Circulatory  system:  A  living  or  newly  killed  specimen  is  somewhat  better 
for  this.  Discover,  if  possible: 

Dorsal  vessel  (just  dorsal  to  the  digestive  tract). 
Ventral  vessel  (just  ventral  to  the  digestive  tract). 


224  ZOOLOGY 

Hearts,  transverse  vessels  connecting  the  longitudinal  vessels,  in  segments 
VII  to  XL 

There  are  other  vessels  more  difficult  to  find.  Examine  a  drop  of  the  con- 
tents of  the  blood  vessels  with  the  microscope. 

(c)  Reproductive  System:  These  organs  are  rather  too  complicated  for  satis- 
factory results  in  an  elementary  class.     Instead  of  a  detailed  examination  note  the 
reproductive  segments   (in  the  region  of  the  esophagus)  with  the  whitish  bodies 
showing  at  the  sides  of  the  alimentary  canal,  and  ventral  to  it.     They  are  attached 
to  the  septa.     (Compare  figures  in  various  text-books.)     Make  a  composite  dia- 
gram of  your  own. 

(d)  Nervous  System:  In  a  well-hardened  preparation,  identify: 

Brain,  a  whitish  lobed  ganglion  just  dorsal  to,  and  in  front  of  the  pharynx; 
collar,  around  the  mouth,  connecting  the  brain  with  ventral  ganglia,  the 
first  of  a  longitudinal  chain  of  ganglia  which  give  off  nerves  in  each  seg- 
ment. How  are  the  ganglia  of  the  ventral  chain  related  to  the  dissepi- 
ments ? 

(e)  Excretory  Organs:  Just  lateral  to  the  nerve-chain  the  student  may  be 
able  to  find  coiled  thread-like  structures  (nephridial  tubes)  in  nearly  all  the  body 
segments  (see  text,  §270).     How  many  in  each  segment? 

5.  Microscopic  Demonstration. — The  teacher  should  make  or  secure  good,  per- 
manent mounts  of  transverse  sections  of  the  earthworm,  by  means  of  which  the 
students  should  identify  the  structures  studied  in  the  gross  dissection,  and  make 
out  more  exactly  the  nature  of  the  following  parts  (see  Fig.  103) : — 

Cuticle,  or  outer  layer. 

Body  wall,  and  the  relation  of  the  circular  and  longitudinal  muscles. 

The  ventral  nerve-chain  in  position. 

The  dorsal  and  ventral  blood  vessels. 

The  wall  of  the  digestive  tract;  gland  cells,  typhlosole,  etc. 

259.  Dero  (or  other  minute  aquatic  Annelid). — Any  one  of 
these  fresh -water  worms  may  be  used  very  profitably  to  sup- 
pliment  the  students'  work  on  the  earthworm.  Mount  the 
living  worm,  being  careful  to  support  the  cover-glass.  Study 
with  low  power.  Compare  at  all  points  with  the  earthworm. 
Dero  may  usually  be  had  at  any  season  of  the  year  by  taking 
mud  and  organic  matter  from  the  bottoms  of  foul  brooks  or 
ponds  and  placing  it  in  vessels  in  the  laboratory.  The  worms 
will  usually  come  to  the  sides  of  the  vessels  where  they  may  be 
seen.  Owing  to  its  transparent  qualities,  such  a  form  will  be 
especially  valuable  in  giving  the  student  a  better  idea  of  the 
performance  of  function  in  the  group.  What  evidences  of  mus- 
cular action  are  manifest  ?  How  is  locomotion  effected  ?  Posi- 
tion and  mode  of  action  of  setae?  Study  the  capture  of  food; 
how  is  its  progress  through  the  digestive  tract,  and  its  elimina- 
tion therefrom  effected?  Do  you  discover  any  circulation  of 


ANNULATA  225 

the  blood?  Direction  of  flow?  Evidences?  How  accom- 
plished? Test  for  ability  to  receive  and  respond  to  stimuli  of 
different  sorts.  Where  are  new  segments  formed?  Discover, 
if  possible,  instances  of  fission,  by  which  new  individuals  are 
formed. 

260.  The  Leech. — The  leech  may  be  studied  and  compared  with  the  earthworm 
as  to  its  external  features,  its  habits,  mode  of  locomotion,  and  the  like.     If  large 
specimens  can  be  had  some  members  of  the  class  might  substitute  it  for  the  earth- 
worm and  the  results  of  the  studies  brought  into  comparison. 

261.  Nereis. — If  specimens  of  Nereis  can  be  obtained  this  worm  should  be 
compared  with  the  earthworm.     (Even  two  or  three  good  specimens  may  be  made 
useful  from  year  to  year  as  comparative  demonstration  both  of  external  and  inter- 
nal structure.) 

Note  especially: 

(a)  The  specialization  of  the  anterior  end;  proboscis,  mouth,  jaws,  palps,  cirri, 
eyes,  antennae. 

(&)  The  fleshy  supports  of  the  bristles,  parapodia. 

DESCRIPTIVE  TEXT 

262.  The  Annulata  are  separated  from  the  unsegmented 
worms  by  the  possession  of  a  series  of  segments  or  metameres 
which  show  on  the  exterior  as  rings,  and  contain  similar  or 
homologous  organs  or  similar  portions  of  a  continuous  organ. 
There  is  also  a  more  uniform  development  of  the  ccelom  than 
in  the  lower  worms.     They  differ  from  the  ccelenterata  in  hav- 
ing bilateral  rather  than  radial  symmetry  in  the  adult  condition. 
The  development  is  often  direct,  but  in  many,  especially  the 
marine  forms,  there  is  a  metamorphosis.     The  larva  of  these 
has  a  peculiar  balloon-shaped  form,  known  as  the  trochophore 
(Fig.  1 06,  £),  similar  in  some  respects  to  the  Rotifers. 

263.  General  Characters. 

1.  Body  elongated,  bilaterally  symmetrical  and  segmented. 

2.  External   paired    appendages    (setas,    bristles,    etc.)    not 
jointed. 

3.  There  is  usually  a  well -developed  body  cavity. 

4.  The    excretory    organs    are   typically    paired    nephridial 
tubules,  one  pair  in  each  segment,  connecting  the  body  cavity 
with  the  outside.     Certain  highly  modified  pairs  of  these  serve 

as  outlets  for  the  reproductive  bodies. 
15 


226 


ZOOLOGY 


•  5-  The  nervous  system  consists  of  (i)  a  supra-esophageal 
ganglion  (brain),  and  (2)  a  circum-esophageal  collar  or  con- 
nective uniting  it  with  (3)  a  ventral  chain  of  ganglia  with  a 
ganglion  in  each  segment. 

6.  Locomotion  is  primarily  effected  by  means  of  the  contrac- 
pIG   10I  tionsof  the  body  wall,  acting  on  body  fluids 

in  the  cavity  within. 

7.  Development  may  be  either  direct 
or  indirect.  When  indirect,  the  larva 
passes  through  a  trochophore  stage. 

264.  General  Survey. — The  Annul ata 
though  conforming  to  the  type  outlined 
above  are  very  diverse  in  appearance, 
habits  and  internal  structure,  While  the 
Chaetopoda, — the  class  to  which  the  forms 
studied  in  the  laboratory  belong, — are 
taken  as  the  type,  the  leeches,  which  have 
no  bristles  but  possess  suckers,  are  un- 
doubtedly related,  as  is  shown  by  their 
development.  The  Rotifers  and  other 
forms  are  sometimes  included  among  the 
relatives  of  the  Annulata.  Metamerism 

FIG.  loi.  Dero,  a.  fresh-water  oligochaetous  annelid,  in  optical 
(frontal)  section.  Enlarged  30  times,  a,  appendages;  br.,  brain; 
d,  dissepiments;  *,  intestine;  m,  mouth;  nph,  nephridium;  oe\ 
esophagus;  p,  pavilion,  lined  with  ciliated  entoderm;  ph.,  pharynx. 
pr.,  processes  from  the  anal  segment;  z,  zone  immediately  in 
front  of  the  anal  segment  where  new  segments  are  continually 
being  formed;  z',  the  zone  of  fission  or  budding.  This  takes 
place  in  the  middle  of  a  segment.  The  anterior  half-segment 
of  z'  will  produce  a  region  like  z  for  the  anterior  half  of  the 
worm.  The  posterior  half-segment  will  produce  a  head  and  four 
segments  like  those  which  contain  the  pharynx  (1-4)  of  the 
parent  worm. 

Questions  on  the  figure. — What  regions  of  the 
digestive  tract  are  sufficiently  differentiated  to  de- 
serve notice?  What  is  the  number  of  the  segment 
in  which  fission  is  taking  place?  What  structures 
must  the  anterior  half  of  this  segment  make?  The 
segment  behind  the  dividing  segment  becomes  num- 
ber 5  of  the  new  posterior  worm.  What  structures 
then  must  be  developed  from  the  posterior  half  of  the 
dividing  segment? 


ANNULATA 


227 


in  animals  is  a  most  interesting  phenomenon  to  zoologists.  This 
group  is  the  first  in  which  we  have  found  true  metamerism.  The 
body  of  the  animals  is  more  or  less  constricted  on  the  outside  into 
rings — as  the  name  (Annulata)  implies.  The  internal  organs  also 
show  metamerism,  but  in  various  ways.  These  organs  may 
pass  directly  through,  with  slight  segmental  modification,  as  the 
digestive  tube  and  ventral  nerve  cord;  they  may  be  repeated 
independently  in  each  segment,  as  the  setae  or  nephridial  tubules ; 
or  they  may  be  represented  in  only  one  or  a  limited  number  of 


FIG.  102. 


C.  V. 


d.  rrv 


n.  c. 


n.  f. 


co. 


FIG.  102.  Longitudinal  section  of  anterior  end  of  Dero.  A,  sagittal  section;  B,  frontal  section 
to  show  anterior  portion  of  nervous  system.  &,  brain;  co.,  nervous  collar  about  the  mouth;  c.v., 
contractile  blood  vessels  ("hearts");  d,  dissepiment;  d.m.,  dermo-muscular  wall;  d.v.,  dorsal  blood 
vessel;  m,  mouth;  n.c.,  nerve  cells;  n./.,  nerve  fibres;  np.,  nephridia;  p,  prostomium;  ph.,  pharynx; 
5,  setae;  sn.,  segmental  nerves;  v.g.  ventral  chain  of  ganglia;  v.v.,  ventral  blood  vessel.  Notice  that 
only  a  portion  of  the  blood  vascular  system  is  shown,  and  this  appears  unsectioned  in  the  figure; 
also  that  nervous  structures  and  nepridia  are  only  partially  represented. 

Questions  on  the  figure. — Compare  this  with  the  cross  section  of  Dero  and 
identify  the  parts.  How  do  the  four  anterior  segments  differ  from  the  others 
figured?  Does  the  ventral  nerve  cord  continue  the  whole  length  of  such  an  animal 
as  this?  Which  organs  may  be  described  as  segmental  and  which  as  continuous 
through  the  segments? 


segments,  as  the  brain  or  the  reproductive  bodies.  The  seg- 
ments are  not  therefore  exactly  equivalent,  yet  the  agreement 
between  successive  segments  is  sufficient  to  merit  the  term 
homonomous  (see  §125).  The  number  of  segments  varies  from 
a  few  to  hundreds.  The  body  is  from  four  or  five  to  many  times 


228 


ZOOLOGY 


as  long  as  broad,  and  is  usually  cylindrical  or  flattened  dorso- 
vent  rally. 

265.  The  dermo -muscular  sac  is  composed  of  the  integu- 
ment or  skin  and  the  muscular  layers  of  the  body  wall.  Being 
filled  with  the  body  fluids  it  is  a  very  important  instrument  of 
locomotion.  This  is  accomplished  by  the  alternate  contractions 
of  the  circular  and  longitudinal  fibres  with  which  the  wall  is 
supplied.  Externally  there  is  a  cuticula,  usually  very  thin, 
overlying  and  secreted  by  the  layer  of  epidermal  cells.  Some 
of  the  cells  of  the  epidermal  layer  are  glandular  and  others  are 
sensory.  The  setae  or  bristles  are  secretions  of  the  epidermal 

FIG.  103. 


c — I 


FIG.  103.  Transverse  section  of  Dero.  X  300.  c.,  ccelom;  c.L,  cells  of  the  so-called  "lateral 
line;"  d.m.,  dermo-  muscular  wall  including  muscles  and  skin;  d.v.,  dorsal  bloodvessel;  «*,  ecto- 
derm; ent,  entoderm;  g,  gut;  g.  /.,  giant  nerve  fibres;  gl,  glandular  cells  assisting  in  digestion;  m.c., 
circular  muscle  fibres;  m.,  longitudinal  muscle  fibres;  n,  nephridium;  n.v.,  ventral  nerve  chain, 
made  up  of  nerve  cells  and  nerve  fibres;  5,  setae;  v.v.,  ventral  blood  vessel. 

Questions  on  the  figure. — Compare  this  with  Fig.  102  and  identify  all  the  struc- 
tures which  appear  in  both.  What  elements  enter  into  the  dermo-muscular  wall? 
Identify  nerve  cells,  fibres  and  the  "giant  fibres"  in  the  ventral  nerve  cord. 

cells  and  lie  in  sacs  in  the  skin.  These  structures  vary  in  num- 
ber and  position  but  are  usually  paired, — two  or  four  groups  to 
each  segment.  They  are  absent  in  the  leeches.  Next  to  the 
skin  is  a  layer  of  circular  muscle  fibres,  and  within  these  are  the 
longitudinal  bands  of  muscle  fibres.  In  the  leeches  there  are 


ANNUL  AT  A  2  29 

also  dorso- ventral  fibres.  Special  groups  of  fibres  occur  in 
connection  with  the  setae,  the  mouth  parts,  suckers,  etc.  The 
fibres  in  worms  are  spindle-shaped  and  unstriate.  The  dermo- 
muscular  wall  bounds  a  true  body  cavity  in  the  chaetopods;  but 
in  leeches  the  ccelom  is  almost  filled  with  connective  tissue. 
This  suggests  the  condition  in  many  of  the  unsegmented  worms. 
See  Figs.  101,  103. 

266.  Worms  as  a  rule  have  no  external  skeleton  other  than 
the  cuticle,  but  in  some  instances  a  tubular  protective  structure 
is  formed  by  secretion  or  by  cementing  together  small  particles 
of  foreign  matter.     Because  of  the  absence  of  hard  skeletal 
parts  little  is  known  concerning  the  worms  of  past  geological 
ages. 

267.  Digestive  System  and  Feeding. — The  stomodaeum,  the 
mesenteron,  and  proctodaeum  (see    §93)  are   all   to  be  distin- 
guished in  the  enteric  canal.     The  mouth  is  not  quite  terminal, 


-v.v. 

FIG.  104.  Transverse  section  of  the  intestine  of  the  Earthworm,  ty,  typhlosole,  an  infolded 
longitudinal  ridge  in  the  gut  in  which  the  gland  cells  feZ.)  are  especially  aggregated.  Other  letters 
as  in  Fig.  103. 

Questions  on  the  figure. — Of  what  conceivable  gain  is  the  typhlosole?  What 
is  it  analogous  to  in  the  higher  types  of  animals? 

but  slightly  ventral.  The  prostomium  (or  preoral  lobe),  a  mus- 
cular extension  of  the  oral  segment,  overarches  it.  There  is 
typically  an  enlarged  muscular  pharynx  which  is  often  eversible, 
followed  by  a  narrow  tubular  esophagus.  Often  there  is  no 
further  differentiation,  the  remainder  of  the  tube  being  fairly 
uniform  and  called  the  intestine.  Frequently,  however,  special 


230  ZOOLOGY 

enlargements  occur,  chief  among  which  is  the  stomach  or  gizzard, 
— a  grinding  organ.  In  the  leeches  the  alimentary  system  is 
much  modified  in  accordance  with  the  blood-sucking  habit  of 
the  animal.  The  crop  is  capable  of  great  enlargement  and  may 
contain  enough  blood  to  nourish  the  animal  for  a  long  time. 
The  mouth  is  sometimes  armed  with  special  cuticular  outgrowths 
which  serve  as  teeth.  Glands  either  unicellular  or  compound 
occur  in"  various  regions  of  the  digestive  tract.  These  secrete 
ferments  similar  to  those  of  higher  animals.  In  the  earthworm 
and  related  forms  there  is  a  dorsal  longitudinal  fold  of  the  in- 
testinal wall  into  the  lumen  of  the  tube,  thus  increasing  the  ex- 
posed surface.  This  is  called  the  typhlosole  (Fig.  104)  and  is 
supplied  with  cells  which  have  been  described  as  digestive. 
The  entodermal  epithelium  may  secrete  a  cuticle  or  may  be 
ciliated.  This  layer  is  surrounded  by  connective  tissue  and 
muscular  fibres. 

Worms  make  use  of  every  possible  kind  of  food.  Earth- 
worms eat  pieces  of  vegetable  or  animal  matter,  and  soil  con- 
taining these.  They  are  lovers  of  darkness,  and  capture  their 
food  at  night.  When  the  light  is  dim  they  may  come  to  the  sur- 
face of  the  ground  and,  with  the  tail  segments  inserted  in  the 
opening  of  the  burrows,  explore  the  surroundings  as  far  as  they 
can  reach.  After  night  showers  they  may  come  entirely  out. 
It  is  at  such  times  as  these  that  they  become  the  prey  of  the 
"early  bird."  Many  of  the  marine  forms  are  actively  car- 
nivorous. Most  leeches  are  blood-suckers,  though  some  are 
carnivorous  or  parasitic.  Many  small  worms  flourish  in  foul 
water  where  they  use  much  decaying  organic  matter  and  micro- 
scopic organisms. 

268.  Respiration. — The  exchange  of  gases  is  effected  for  the 
most  part  through  the  general  body  wall,  into  which  the  blood 
capillaries  or  the  lacunae  of  the  coelom  may  penetrate.     In  some 
forms  there  are  special  thin  places  and  out-pocketings  of  the  body- 
wall  (branchiae)  by  which  the  exchange  is  facilitated.     These 
are  characteristic  of  the  Polychaeta  especially  (Figs.  107,  108). 

269.  Circulation. — In  some  of  the  simplest  worms  there  are 
no  special  blood  vessels.     The  ccelomic  spaces  contain  a  fluid, 


ANNUL  AT  A  231 

which  possesses  corpuscles  and  is  moved  by  the  general  body 
contractions.  In  the  typical  condition  there  are  two  or  more 
longitudinal  vessels,  dorsal  and  ventral  (or  lateral)  in  position. 
These  are  often  connected  by  transverse  loops  in  a  few  or  many 
segments  of  the  body  especially  at  the  anterior  and  posterior 
ends.  The  circum-intestinal  loops  are  often  contractile,  and 
the  longitudinal  vessels  may  show  a  wave  of  contraction  passing 
from  one  end  to  the  other.  Capillaries  vary  much  in  perfection 
of  development.  The  blood  contains  only  white  corpuscles. 
There  is  haemoglobin  in  the  plasma. 

270.  Excretion  takes  place  by  means  of  the  segmental  organs 
or  nephridia,  of  which  there  is  usually  one  pair  in  each  segment, 
with  the  exception  of  some  of  the  anterior  segments.     The 
nephridium  is  a  tubular  structure  consisting  essentially  of  the 
following  portions  (Fig.  35):  (i)  a  ciliated  funnel,  communi- 
cating with  the  ccelom;  (2)  a  tortuous  glandular  region;  and  (3) 
an  outlet  through  the'  body  wall,  often  supplied  with  muscle 
fibres.     The  nitrogenous  waste  products  find  their  way  into  the 
fluid  of  the  ccelom  and  thence  into  the  nephridium,  or  directly 
into  the  nephridium  from  blood  capillaries  which  may  occur  in 
its  walls,  and  thus  are  finally  eliminated  upon  the  exterior  of 
the  body. 

271.  Nervous  System  and  Behavior. — The  "central"  nerv- 
ous system  may  be  said  to  consist  of  three  portions:  (i)  a  mid- 
ventral  line  of  nerve  fibres,  and  nerve  cells  which  are  diffusely 
scattered  or  collected  in  ganglia,  (2)  a  brain  which  is  anterior 
and  dorsal  to  the  pharynx,  (3)  a  connective,  or  collar  about  the 
pharynx  connecting   (i)   and   (2)    (Fig.   102).     The  brain  and 
ventral  cord  may  show  distinct  right  and  left  lobes  or  may  be 
completely  fused  into  a  median  mass.     From  the  brain,  nerves 
pass  to  the  'head-parts.     From  each  of  the  segmental  portions 
of  the  ventral  chain  nerves  pass  to  the  walls,  viscera,  etc.     The 
ventral  cord  frequently  lies  in  a  blood  sinus  which  secures  its 
abundant  nourishment  (leeches). 

The  sense  organs  occur  very  unequally  in  the  group.  The 
Polychaeta  and  the  leeches  are  best  supplied.  The  skin  is 
generally  sensitive  to  mechanical  contact,  to  chemical  stimuli 


232  ZOOLOGY 

and  to  moisture.  This  sensitiveness  is  perhaps  specially  local- 
ized in  the  tentacles,  cirri,  and  more  movable  parts.  Statocysts, 
fluid-filled  cavities  bounded  by  sensory  epithelium,  occasionally 
occur  (see  §m).  Some  solid  particles  (statoliths)  float  in  the 
fluid.  These  have  been  described  as  organs  of  hearing  but  the 
sensation  resulting  is  probably  quite  different  from  what  we 
know  as  hearing.  They  are  apparently  organs  of  equilibration, 
enabling  the  animal  to  appreciate  its  position  in  relation  to  the 
pull  of  gravity  and  to  appreciate  the  action  of  water  waves. 
Eyes  may  consist  merely  of  a  group  of  pigmented  cells  with 
nervous  connections,  or  may  be  very  complicated,  consisting 
of  a  capsule  with  refractive  media  and  retina.  Images  of  ob- 
jects are  not  formed,  in  all  probability,  but  the  direction  and 
intensity  of  light  can  be  appreciated.  In  the  leech  there  are 
sense  organs  in  each  segment  somewhat  similar  in  structure  to 
the  eyes.  Their  function  is  unknown.  Even  in  the  earthworm 
and  other  forms  in  which  there  are  no  eyes,  the  skin  is  sensitive 
to  light.  Most  worms  prefer  dark  places. 

272.  Reproductive  Organs. — The  Oligochaeta  and  the  leeches 
are  hermaphrodite.  In  the  Polychaeta  the  sexes  are  separate. 
The  sexual  products  are  developed  from  the  ccelomic  epithelium, 
sometimes  on  the  dissepiments,  sometimes  on  the  body  wall,  or 
in  other  special  regions.  The  elements  may  be  produced  in 
many  segments  (Polychaeta),  or  in  a  few  anterior  ones  (Oligo- 
chaeta). The  region  is  usually  distinguishable  only  about  the 
breeding  time.  In  the  hermaphrodite  forms  the  ova  and  sper- 
matozoa often  mature  at  different  times  and  are  produced  in 
different  segments.  This  of  course  insures  cross-fertilization. 
In  the  Polychaeta  the  conditions  are  relatively  simple.  The 
elements  are  freed  in  the  body  cavity  and  when  mature  find 
their  way  into  the  water  where  fertilization  takes  place.  The 
organs  are  much  more  complicated  in  the  hermaphrodite  worms. 
The  spermatozoa  are  produced  in  the  testes,  are  passed  into  the 
seminal  vesicles  where  they  are  matured,  and  at  the  time  of 
copulation  escape  to  the  exterior  by  the  vasa  deferentia,  to  be 
deposited  in  the  sperm  sacs  or  receptacula  seminis  of  another 
worm.  From  this  place,  any  time  after  copulation,  the  sperm 


ANNUL  AT  A 


233 


is  brought  into  contact  with  the  ova  of  this  second  worm,  as 
they  pass  from  the  ovary,  where  they  are  produced,  to  the  egg-sac 
or  to  the  exterior.  It  is  believed  that  in  some  instances  at  least 
the  genital  ducts  are  modified  nephridia. 

It  will  be  a  profitable  practical  exercise  for  the  student  to 
make  for  himself  a  diagram  of  the  sex  organs  of  the  earthworm 
or  some  other  hermaphrodite  form  by  reference  to  several  stand- 
ard texts. 

273.  Reproduction  and  Development. — Sexual  reproduction 
is  universal.  As  we  have  seen,  copulation  may  occur  or  the 
elements  may  come  together  in  the  water.  In  the  Oligochaeta 


xmi.3 


FIG.  105.  Two  stages  in  the  development  of  Nereis.  A,  8-celled  stage;  B~,  lo-celled  stage, 
both  viewed  from  the  active  or  ectodermal  pole,  mi.1,  mi,*,  and  mi.*,  the  first,  second  and  third 
sets  of  micromeres  separated  from  ma.,  the  macromeres;  s1,  first  somatoblast,  one  of  the  second 
group  of  four  cells  to  be  budded  from  the  macromeres;  s2,  second  somatoblast,  one  of  the  third  group, 
which  gives  rise  to  the  mesoderm.  The  micromeres  are  ectodermal  and  the  macromeres  produce 
the  entoderm.  (After  Westinghausen.) 

and  leeches  the  fertilized  ova,  or  the  ova  together  with  masses 
of  spermatozoa,  are  enclosed  in  a  cocoon  of  secreted  material 
and  within  this  case  the  young  worm  is  developed.  In  the 
Polychaeta  the  larva  undergoes  its  development  in  a  free  state. 
Segmentation  in  Annulata  is  complete  and  usually  unequal, 
giving  rise  at  the  eight- celled  stage  to  four  micromeres  and  four 
macromeres  (Fig.  105).  The  micromeres  produce  the  ectoderm; 
directly  or  indirectly  the  macromeres  give  rise  to  the  entoderm. 
Early  in  the  cleavage  "primitive  mesoblasts" — cells  which  pro- 
duce the  mesodermal  structures — are  separated  from  the  macro- 
meres. A  gastrula  is  formed  either  by  invagination  or  by  over- 


234 


ZOOLOGY 


growth.  In  the  earthworm  (Oligochaeta)  the  blastopore  of  the 
gastrula  forms  the  mouth  of  the  adult  worm.  In  Nereis  (Poly- 
chaeta)  the  blastopore  closes  by  growth,  and  the  stomodaeum 
and  proctodaeum  arise  by  ectodermic  in  vagi  nations  which  finally 
become  continuous  with  the  entoderm  of  the  archenteron  (Fig. 
106,  D,  of  Polygordius).  A  ciliated,  free-swimming  larval  stage 


-S.C.. 


st. 


FIG.  106.  Diagrams  of  stages  in  the  metamorphosis  of  Polygordius,  a  primitive  annelid.  Ecto- 
derm throughout  is  represented  as  nucleated  without  cell  boundaries;  the  entoderm  has  the  cell- 
boundaries  shown,  and  the  mesoderm  is  diagonally  shaded.  A,  gastrula;  B,  same  with  blastopore 
closed;  C  and  D  represent  formation  of  stomodaeum  and  proctodaeum  from  ectoderm;  E,  Tr echo- 
sphere  stage  showing  formation  of  segments  in  the  posterior  portion;  F,  adult  (sagittal):  G,  adult 
(transverse),  a,  archenteron;  bp.,  blastopore;  br,  brain;  c,  coelom;  d,  dorsal;  di,  dissepiments;  m, 
inesenteron;  pr.,  proctodaeum;  s.c.,  segmentation  cavity;  st,  stomodaeum;  v.n.,  ventral  nerve  chain; 
t,  zone  of  formation  of  nerve  segments.  (After  Fraipont.) 

Questions  on  the  figures. — Trace  the  behavior  of  ectoderm  and  entoderm  in 
these  figures  and  determine  what  structures  each  seems  to  give  rise  to.  What  is  a 
trochosphere?  Distinguish  between  somatic  (body)  and  splanchnic  mesoderm. 
(See  §58.) 

ensues, — known  as  a  trochosphere  (Fig.  1 06,  E) .  The  trochosphere 
may  be  looked  upon  as  representing  the  anterior  or  head  end 
of  the  adult.  The  later  metamorphosis  to  the  adult  condition 


ANNUL  ATA  235 

involves  the  reduction  in  size  of  the  enormous  anterior  region, 
and  the  growth  of  segments  at  the  posterior  end,  and  is  char- 
acteristic of  Polychaeta.  The  development  of  leeches  is  direct 
as  in  the  Oligochaeta,  or  in  some  instances  it  might  be  more 
accurate  to  say  that  the  process  of  metamorphosis  is  very  much 
abbreviated,  being  completed  by  the  time  of  hatching. 

274.  In  addition  to  sexual  reproduction  many  worms,  par- 
ticularly the  aquatic  forms,  have  the  power  of  multiplying  by 
fission.     In  some  instances  this  may  consist  of  a  mere  breaking 
in  two,  as  was  seen  to  be  possible  in  the  starfish, — each  part 
regenerating  segments  corresponding  to  those  lost.     In  other 
cases  (Nais,  Dero,  etc.)  zones  of  rapidly  forming  segments  are 
produced  somewhere  in  the  mid-region  of  the  body,  and  from  this 
zone  a  new  head  is  developed  for  the  posterior  zooid  and  a  new 
tail  for  the  anterior  zooid,  which  usually  become  structurally 
complete  before  the  separation  takes  place  (Fig.  101,  z'). 

In  some  of  the  Polychaeta  (as  Autolytus)  a  distinct  alternation 
of  generation  is  found  in  which  sexual  and  non-sexual  individuals 
are  of  very  different  appearance. 

When  artificially  mutilated  the  earthworm,  and  some  other 
types  as  well,  may  regenerate  the  lost  portions.  Groups  of 
segments  of  one  worm  may  be  grafted  upon  another,  complete 
healing  taking  place  in  such  a  way  as  to  produce  an  apparently 
normal  worm.  Pieces  may  be  grafted  on  the  side  of  another 
worm  in  such  a  way  as  to  produce  a  forked  or  otherwise 
abnormal  result. 

275.  Ecology. — The    leeches    are    aquatic    in    habit    and 
many  of  them  live  on  the  blood  of  higher  animals, — a  kind 
of  temporary  parasitism;  the  Polychaeta  are  marine,  and  the 
Oligochaeta  are  chiefly  fresh  water  or  terrestrial  in  habit.     A 
few  of  the  latter  groups  are  parasitic.     Of  the  aquatic  worms 
some   are   actively   free-swimming,    others   crawl   in   and   out 
among  the  living  and  dead  matter  of  the  bottom,  others  bur- 
row in  the  sand,  or  secrete  a  tubular  skeleton  into  which  they 
may  retire.     Their  chief  economic  importance  is  that  they  serve 
as  food  for  fish  and  other  food-animals.     The  earthworm,  in 
forming  its  underground  burrows,  eats  its  way  into  the  earth, 


236 


ZOOLOGY 


swallowing  the  soil  for  the  organic  matter  which  it  contains 
and  passing  it  through  its  digestive  tract.  These  castings 
may  often  be  seen  at  the  mouth  of  the  burrows.  Worms  thus 
break  up  the  soil,  making  it  more  porous  and  accessible  to  air, 
moisture,  bacteria,  and  the  rootlets  of  plants.  Darwin  esti- 
mates that  three  inches  of  the  subsoil  is  thus  brought  to  the 
surface  in  fifteen  years  through  this  agency.  Doubtless  earth- 
worms bring  to  the  surface  materials  that  renew  the  soil  fertility, 
replacing  substances  taken  from  the  surface  soil  by  plants. 

276.  Classification. 

Class  I.  ChcBtopoda  (bristle-footed}. — Annulata  with  metameres  usually  well 
marked  both  externally  and  internally;  with  setae  developed  from  the  epidermis. 
The  co3lom  is  usually  voluminous  and  is  divided  into  chambers  by  transverse  dis- 
sepiments. Closed  blood-vascular  system.  Ventral  nerve-chain  ordinarily > with 
a  distinct  ganglion  to  each  segment. 

FIG.   107. 

^a^jm^ 


PIG.   107.     Amphitrite  ornata,  from  Yen-ill's  "Invertebrate  Animals  of  Vineyard  Sound.  ' 

Subclass  I.  Polychata  (with  numerous  bristles}. — Marine  Chaetopoda  with 
numerous  setae  typically  borne  on  elevations  of  the  body  wall  (parapodia).  Head 
usually  well  differentiated,  bearing  eyes,  antennae,  cirri,  etc.  Branchiae  or  gills 
often  present.  Sexes  separate;  the  reproductive  organs  simple,  and  repeated  in 
many  segments.  A  metamorphosis  occurs;  the  larva  is  known  as  a  trochosphere. 

Nereis,  the  "sand  worm"  of  fishermen,  is  a  type  of  this  group.  Autolytus  is  a 
small  worm  especially  interesting  because  of  its  power  of  reproducing  by  fission. 
The  bud  which  is  freed  from  the  hinder  end  of  the  worm  differs  from  the  parent 


ANNULATA 


237 


stock  in  that  it  is  sexual.  Amphitrite  is  a  beautiful  worm  which  represents  the 
attached  or  tube-forming  types.  As  the  result  of  their  habits  such  forms  tend  to 
lose  their  segmentation  and  the  appendages  of  the  posterior  part  of  the  body.  The 
gills  and  tentacles  accumulate  about  the  head.  These  and  other  types  grow  abun- 
dantly in  the  sand  and  mud  of  harbors,  amid  the  vegetation  of  the  bottom,  and  over 
exposed  objects  of  all  sorts  from  low-water  mark  to  unknown  depths.  Their 
value  in  utilizing  debris  and  the  more  minute  organisms  as  food  and  thus  becoming 
a  link  in  the  saving  of  these  to  serve  as  food  for  the  higher  organisms  cannot  be 
overestimated.  (Figs.  107  and  108.) 

Subclass  II.  Oligochata  (with  few  bristles'). — These  are  Chaetopoda  with  no 
parapodia  and  comparatively  few  setae  which  usually  occur  in  two  or  four  clusters 
in  each  segment.  They  are  mostly  fresh  water  or  terrestrial  in  habit.  Fleshy 

FIG.   108. 


FIG.    108.      Cirratulus  grandis,  from  Verrill. 

Questions  on  Figs.  107  and  108. — Are  these  Chaetopods?  What  are  your 
evidences?  What  is  the  nature  and  function  of  the  numerous  outgrowths  (bran- 
chial cirri)?  In  what  respects  are  they  differently  arranged  in  the  two  types? 
Are  these  Oligochaeta  or  Polychasta?  Your  reasons? 

outgrowths,  such  as  gills,  are  almost  universally  absent.  The  sexes  are  united  in 
one  individual  and  the  accessory  reproductive  organs  are  very  complicated.  Ova- 
ries and  testes  limited  to  a  small  number  of  anterior  segments;  development 
direct.  The  head  not  so  highly  specialized  as  in  the  Polychaeta. 

The  earthworms,  of  which  there  are  numerous  species,  are  the  best  known  types 
of  this  subclass.  The  genera  and  species  are  distinguished  chiefly  by  the  position 
of  the  sexual  organs.  The  aquatic  Oligochaeta,  which  are  much  smaller,  are  found 
in  practically  all  ponds  and  ditches  where  organic  matter  is  decaying.  The  more 


238  ZOOLOGY 

common  genera  are  Dero  (Fig.  101),  a  beautiful,  almost  transparent  worm  which 
often  forms  a  temporary  tube  for  itself  of  particles  cemented  by  its  own  secretion, 
and  Tubifex,  a  longer  worm  which  burrows  in  the  mud  at  the  bottom  of  streams; 
a  portion  of  the  body  protrudes  from  the  mud  and  waves  gently  back  and  forth 
in  the  water.  They  may  occur  so  thickly  that  thousands  may  be  seen  in  the  space 
of  a  few  feet.  When  their  home  is  jarred  they  speedily  withdraw  from  sight.  A 
colony  of  Tubifex  nearly  always  has  associated  with  it  one  or  more  genera  of 
smaller  worms,  as  Dero  or  Nais,  a  species  similar  to  Dero  but  with  eyespots.  Dero 
has  an  interesting  respiratory  apparatus  at  the  posterior  part  of  the  body  (Fig. 
101,  p.),  one  of  the  few  instances  where  Oligochaeta  possess  such  organs. 

Class  II.  Discophora  (bearing  suckers] . — Annulata  in  which  there  are  secondary 
external  rings  which  tend  to  obscure  the  metameres,  inasmuch  as  the  external 
and  internal  segmentations  do  not  coincide.  There  are  no  bristles.  The  body 
cavity  is  much  reduced  by  the  growth  of  muscles  and  connective  tissue.  The 
remaining  spaces  contain  blood  and  are  in  communication  with  the  vascular 
system.  Two  sucking  discs  are  present  and  are  powerful  organs  of  attachment. 
The  anterior  sucker  embraces  the  mouth;  the  posterior  is  near  the  anus.  Sexes 
are  united  in  one  individual;  testes  numerous,  ovaries  a  single  pair.  Development 
direct.  Marine,  fresh  water,  terrestrial,  or  parasitic  in  habit. 

277.  There  are  several  other  groups  of  "worms"  of  considerable  interest  to  the 
zoologist,  sometimes  associated  with  the  Annulata,  which  it  seems  necessary  to  pass 
by  with  mere  mention.    -(See  also  §236). 

Class:  Archi-annelida ;  a  few  primitive  forms,  as  Polygordius  (Fig.  106). 
Class:  Sipunculoidea  (Gephyrea).     With  traces  of  segmentation  in  the  embryo, 
but  not  in  the  adult. 

Class:  Chaetognatha  (arrow  worms). 

Some  authors  would  place  here  also  the  Rotifers  (see  §235). 

278.  Suggestive  Studies  for  Library  and  Laboratory. 

1 .  Look  up  the  characteristics  of  the  Archi-annelida,  the  Gephyrea,  or  Sagitta 
and  report  on  their  likenesses  to  the  types  studied. 

2.  On  what  grounds  might  the  rotifers  be  associated  with  the  Annulata? 

3.  Compare  the  "segments"  in  cestodes  and  Annulata. 

4.  In  the  Chaetopoda  which  sets  of  organs  pass  through  all  the  segments,  which 
are  repeated  in  essentially  all,  and  which  are  limited  to  a  few? 

5.  Examine  and  report  on  the  habits  of  the  earthworm.     (Study  in  its  natural 
haunts  or  in  box  of  moist  earth  in  laboratory.)     What  are  its  haunts?     Method 
and  rate  of  burrowing?     Does  it  avoid  water?     What  is  its  food?     How  taken? 
Does  the  animal  prefer  light  or  darkness? 

6.  If  near  the  sea-shore  select  other  forms  and  report  in  a  similar  way. 

7.  Investigate  parasitism  among  the  Annulata. 

8.  What  is  the  economic  value  of  the  earthworm?     Of  other  worms? 

9.  Make  a  study  from  the  text-books  of  the  reproductive  organs  in  any  of  the 
hermaphrodite  Oligochaeta. 

10.  In  how  many  species  of  aquatic  Oligochaeta  do  you  find  reproduction  by 
fission  ?     In  what  particulars  does  the  process  seem  to  differ  in  the  different  species  ? 

11.  Outline  the  life-history  of  Autolytus,  including  the  origin  of  the  sexes. 


CHAPTER  XVI 

PHYLUM  X.— MOLLUSCA 
LABORATORY  EXERCISES 

279.  The  Clam  (My a)  or  Mussel  (Anodonta,  Unid). — Either 
the  marine  or  the  fresh-water  type  will  serve.  The  latter  are 
to  be  found  in  almost  all  our  streams  and  small  lakes.  They 
may  be  obtained  with  a  long-handled  rake  from  the  shore  or 
from  a  boat.  They  often  occur  partly  buried  in  the  sand  or 
mud.  If  kept  in  water  they  may  be  transported  to  the  labora- 
tory and  placed  in  a  tub  of  water  with  a  few  inches  of  sand  at 
the  bottom,  where  something  of  the  physiology  may  be  studied 
with  profit.  If  they  cannot  be  collected  when  needed  for  study, 
care  should  be  taken  to  supply  plenty  of  the  preservative  fluid 
in  which  they  are  kept. 

i .  The  Living  Animal. — What  facts  were  observed,  in  col- 
lecting the  material,  concerning  their  haunts,  their  abundance 
in  different  localities,  their  range  in  size,  etc.  ?  Are  there  any 
efforts  at  active  feeding,  as  far  as  you  have  seen?  Do  you 
think  all  your  specimens  belong  to  the  same  species  ? 

In  nature  or  from  the  specimens  in  the  tub,  make  out  the 
following  points: 

Has  the  animal  power  of  voluntary  motion  ?  If  so,  what  of 
its  rate,  manner,  and  the  position  of  the  animal  during  motion  ? 
Note  the  trail.  Determine  anterior  and  posterior  ends,  right 
and  left  sides,  dorsal  and  ventral  surfaces.  To  what  extent 
can  the  soft  parts  protrude  from  the  shell?  Note  briefly,  for 
later  reference,  the  position  of  all  visible  structures.  How 
widely  does  the  shell  open  during  life  ?  With  a  pipette  place  a 
drop  of  some  colored  but  harmless  fluid  (carmine  in  water) 
near  the  fringes  of  the  posterior  end,  and  note  the  results. 
Vary  by  introducing  salt,  sugar,  and  acid  in  the  solution. 
Devise  experiments  to  test  whether  the  animal  shows  sensitive- 

239 


240  ZOOLOGY 

ness  to  stimuli  of  various  sorts:  jars,  contacts,  currents  in  the 
water,  light,  warmth  and  cold. 

2.  General  Form. — Renew  your  observations  concerning  the 
symmetry  of  the  clam  by  careful  examination  of  the  animal. 
Determine  and  show  in  a  sketch  all  the  points  distinguishing 
the  anterior  and  posterior  ends.     Are  the  right  and  left  halves 
symmetrical  ?     Use  a  pair  of  empty  shells  for  completer  study. 

The  shell:  what  is  the  relation  between  the  valves?  How 
are  they  held  together?  Are  they  normally  open  or  closed? 
Give  your  evidences?  To  what  extent  may  the  shell  open 
without  violence?  How  does  the  shell  vary  in  thickness  at 
various  parts  ?  Contrast  the  interior  and  exterior  as  to  finish 
and  markings.  Make  note  of  everything  found,  with  outline 
drawings,  showing  position.  Locate  the  following  regions  and 
structures: — hinge,  umbo  or  beak  (a  prominence  near  the  hinge), 
hinge  ligament,  hinge  teeth,  pallial  line  (a  slight  depression 
marking  the  attachment  of  the  mantle  muscle),  muscle  im- 
pressions, lines  of  growth.  Review  after  studying  soft  parts. 

What  layers  are  discoverable  in  a  broken  shell  ?  How  do 
the  inner,  outer,  and  middle  layers  differ  in  thickness  and 
appearance  ? 

Do  you  find  any  differences  worthy  of  note  in  different 
individuals  ? 

3.  Soft  Parts. — Remove  one  valve  (say  the  left)  by  cutting 
the  two  muscles  which  hold  the  valves  together.     Leave  all 
the  soft  parts  in  the  right  valve  as  little  disturbed  as  possible. 
Make  a  sketch  showing  the  relation  of  the  body  to  the  shell. 
If  there  is  any  difficulty  in  cutting  the  muscles,  the  clam  may  be 
made  to  open  by  immersing  it  in  water  heated  to  about  140°  F. 
Identify : 

Left  mantle  flap.     How  related  to  the  right  ?  to  the  shell  ? 

Siphons :  modifications  of  the  posterior  margins  of  the  mantle. 
(These  will  be  conspicuous  or  rudimentary  in  accordance  with 
the  species  studied.)  Number? 

Adductor  muscles  of  the  valves ;  number  and  position. 

Mantle  cavity.  Separate  the  right  and  left  mantle  lobes 
along  the  ventral  margin,  except  in  the  region  of  the  siphon, 
and  fold  back  the  left.  Where  is  it  attached  to  the  body? 


MOLLUSCA  241 

The  ventral  or  incurrent  siphon  opens  into  the  branchial  chamber, 
the  dorsal  or  eoccurrent  into  a  smaller  dorsal  chamber, — the  cloacal. 
Verify  and  sketch. 

Gill  plates  or  sheets;  number  and  attachment.  Are  they 
symmetrical  on  the  two  sides?  The  eggs  and  developing  em- 
bryos may  be  found  in  the  outer  gill  cavity  at  favorable  times. 
(A  special  study  and  report  may  be  profitably  made  by  some 
student  on  the  structure  of  the  gills  as  shown  by  a  hand  lens 
and  the  low  power  of  the  microscope.  A  bit  of  the  living  gill 
from  a  fresh  specimen  should  be  examined.) 

Abdomen, — the  soft,  fleshy  mass  between  the  pairs  of  gills, 
which  terminates  in  a  more  solid  part, — 

Foot :  position  and  form  ? 

Mouth  and  labial  palps;  at  the  anterior  end  and  just  below 
the  adductor  muscle.  How  many  palps  ? 

(It  is  to  be  remembered  that  all  the  structures  examined  thus 
far  are  external  organs.  The  body  wall  has  not  been  penetrated 
at  all.  If  it  is  the  plan  to  study  the  anatomy  more  closely, 
the  following  are  the  chief  sets  of  organs  deserving  attention.) 

4.  Other  systems  of  organs. 

Circulatory  system.     Open  the  pericardial  cavity,  just  beneath  the  hinge  and 
a  little  posterior  thereto,  find  the 

Heart:  auricles  and  ventricle.     In  a  fresh  preparation  the  contractions  of 

the  heart  may  be  observed. 
Vessels:  one  passes  in  each  direction,  but  they  are  not  easily  seen  without 

injecting. 
The  intestine  passes  through  the  ventricle  without  open  communication 

with  it. 
Excretory  organ. 

Organ  of  Bojanus,  or  kidney,  lies  just  beneath  the  floor  of  the  pericardial 

cavity,  one  part  on  either  side. 

Each  portion  is  a  dark-colored  sac,  with  an  abundant  btood  supply. 
Nervous  system.     (Traced  best  in  hardened  preparations.) 

Visceral  ganglia.  Look  between  the  gills  in  the  posterior  portion  of  the 
body,  beneath  the  posterior  adductor  muscle,  and  in  the  floor  of  the 
cloacal  cavity.  Number,  and  closeness  of  connection?  By  careful  dis- 
section determine  what  nerves  leave  them.  Trace  a  pair  of  these  forward 
to  the 

Cerebral  ganglia,  on  either  side  the  mouth.  Note  the  connections  between 
the  cerebral  ganglia.  Trace  from  these  ganglia  the  connectives  which 
pass  ventrally  to  the 

Pedal  ganglia  in  the  muscular  foot,  close  to  its  union  with  the  abdomen. 
Make  a  clear  diagram  showing  the  relations  of  these  three  pairs  of  ganglia. 
16 


242  ZOOLOGY 

Digestive  system. 

Begin  with  the  intestine  at  the  heart.  Trace  posteriorly  to  the  anus. 
What  is  its  relation  to  the  posterior  adductor  muscle?  Pass  a  bristle 
into  the  intestine  anteriorly  and  use  it  to  guide  the  dissection.  Trace 
the  intestine  through  the  abdominal  mass,  and  plot  its  course.  Identify 
the  stomach,  the  esophagus,  and  the  mouth.  The  liver  is  a  brownish 
or  greenish  mass  surrounding  the  stomach. 
Much  of  the  visceral  mass  through  which  the  intestine  coils  is  made  up  of 

the  large  reproductive  glands  which  open  into  the  mantle  cavity. 
5.  Cross  Sections. — A  series  of  cross  sections  may  be  made  by  the  teacher, 
numbered,  and  used  with  profit  as  demonstrations.  For  such  sections  the  soft 
parts  of  the  animal  should  be  hardened  for  24  hours  in  I  per  cent,  chromic  acid; 
then  one  day  each  in  70  per  cent,  and  90  per  cent,  alcohol.  Keep  in  95  per  cent, 
alcohol  for  a  few  weeks.  Cut  one-fourth  to  one-third  inch  thick  and  number  so 
as  to  be  able  to  locate  position  of  section.  Float  in  dish  of  alcohol  and  identify 
the  parts  found.  Make  sketches  of  sections  passing  (i)  through  the  stomach,  (2) 
through  the  heart,  and  (3)  through  the  middle  of  the  posterior  adductor  muscle. 
In  the  absence  of  these  the  student  should  be  encouraged  to  make  a  diagram  of  an 
imaginary  cross  section  through  the  middle  of  the  body.  Include  the  shell. 

286.  The  Oyster. — One  or  two  students  should  be  asked  to  prepare  a  report  on 
the  structure  of  the  oyster  and  present  to  the  class  an  account  of  the  chief  points 
of  contrast  between  the  oyster  and  the  clam.  The  adult  oyster  is  fixed  by  one 
of  its  valves.  Is  it  the  same  one  in  all  specimens? 

281.  The  Pond  Snail  (Limn&d). 

1.  The  Living  Animal. — Observe,  both  in  its  natural  home 
and  in  glass  vessel  containing  water,  in  the  laboratory. 

To  what  does  the  animal  adhere  in  the  water  ?  Must  it  have 
solid  support?  Can  it  swim?  What  is  its  method  of  locomo- 
tion ?  What  does  it  eat,  and  how  ?  Can  you  determine  whether 
it  uses  the  air  in  breathing  or  gets  its  oxygen  from  the  water  ? 
Proof?  How  is  the  gliding  motion  effected?  Watch,  with  a 
lens,  one  crawling  along  the  side  of  the  glass  vessel.  Record 
signs  of  sensitiveness  to  stimuli,  by  experiments  of  your  own 
devising. 

2.  General  Form. — Is  there  any  sign  of  bilateral  symmetry? 
In  what  parts  ?     How  are  anterior  and  posterior  distinguished  ? 
Relation  of  the  shell  to  the  animal  ?     Identify : — 

Head:  tentacles,  number  and  position;  eyes,  number  and 
position. 

Foot,  the  muscular  expansion:  shape,  changes  in  form  and 
position. 


MOLLUSCA  243 

Mouth. 

Respiratory  orifice,  position.  Under  what  circumstances 
seen? 

3.  Shell  (secure  empty  ones). — Make  sketches  of  the  shell 
and  identify  the  structures  referred  to  in  the  following  terms: 
apex,  aperture,  lip,  spire,  whorl,  suture,  columella.  (See 
Fig.  no). 

How  would  you  describe  the  direction  of  the  spiral?  How 
many  whorls?  Have  the  young  and  old  the  same  number? 
Can  you  detect  lines  of  growth  ? 

4.  Soft  Parts. — These  may  be  removed  by  dropping  the  animal  suddenly  into 
hot  water,  and  then  gradually  twisting  the  soft  portion  from  the  shell.  It  will 
scarcely  repay  the  trouble  to  do  more  than  re-identify  the  following  parts:  mouth, 
respiratory  orifice,  mantle  and  mantle  chamber,  and  collar  (a  portion  of  the 
mantle).  The  spiral  is  occupied  by  the  digestive  tract,  its  glands,  the  reproduc- 
tive bodies,  etc. 

5.  Development. — Examine  the  stems  of  plants  and  the  sides  of  the  vessel 
in  which  snails  have  been  kept  for  some  days  for  gelatinous  capsules  of  eggs.  They 
are  almost  transparent  and  the  eggs  may  be  easily  located.  What  seems  to  be 
the  value  of  the  gelatine?  Number  and  arrangement  of  the  eggs?  What  is  the 
shape  of  the  eggs?  Get  the  earliest  stages  possible,  and  watch  day  by  day  at 
short  intervals,  or  compare  capsules  of  different  ages.  If  care  is  taken,  some  idea 
of  the  early  segmentation  stages  may  be  obtained.  Lookfortheblastula:  are  the 
cells  of  the  same  size?  Do  you  find  a  gastrula?  What  are  the  first  signs  you  find 
of  differentiation  of  parts?  Look  for  different  stages  of  the  later  development. 
It  will  not  be  profitable  to  try  to  follow  the  changes  in  detail. 

282.  A  very  valuable  laboratory  exercise  may  be  had  by  comparing  large 
numbers  of  shells  of  a  single  species,  found  under  varying  conditions.     Compare 
as  to  shape,  markings,  etc.,  and  see  whether  there  are  individuals  connecting  your 
extreme  groups.     The  land  snail  (Helix,  Fig.  121)  is  favorable  for  such  study. 
Helix  may  be  substituted  for  Limnaa. 

283.  The  Squid. — The  teacher  should  at  least  have  a  few  specimens  of  the 
Squid,  from  which  the  pupils  may  be  required  to  get  some  idea  of  the  general  form. 
Drawings  should  be  made,  showing  all  external  features. 

Note  particularly: — 

Head:  tentacles,  number,  comparative  length;  suckers  on  the  inner  surface, 

arrangement  of  suckers. 
Eyes:  number,  size,  position. 

Olfactory  organs  opening  beneath  folds  of  skin  behind  the  eyes. 
Neck. 

Body:  general  shape.     It  is  surrounded  by  the 
Mantle;  note  the  fin  expansions  at  the  posterior  end.     Where  are  the 

attachments  of  the  mantle  to  the  body? 
Siphon;  how  related  to  the  mantle  cavity? 


244  ZOOLOGY 

What  are  your  conclusions  as  to  the  symmetry  and  the  normal  position  of  the 
squid  ?  Do  you  find  anything  from  your  external  examination  which  would  lead 
you  to  class  it  with  the  clam  and  the  snail? 

DESCRIPTIVE  TEXT 

284.  The    group    Mollusca     (mollis,    soft)    embraces    from 
30,000  to  60,000  living  species  among  which  there  are  very  great 
differences,  as  illustrated  by  forms  as  unlike  as  slugs,  snails, 
oysters,  clams,  devil-fishes,  and  squids.     With  the  exception  of 
a  few,  they  are  sluggish  animals  and  largely  aquatic  or  fre- 
quenters of  moist  places.     Some  are  well  protected  by  external 
armor   and   others   are   perfectly   naked.     The   typical   adult 
mollusk  is  clearly  marked  off  from  both  the  radiate  animals 
such  as  echinoderms  and  the  segmented  animals  such  as  the 
arthropods  and  the  Annulata,  but  some  of  the  simpler  types  of 
mollusks,  and  the  larvae  of  certain  of  them  which  undergo  a 
metamorphosis,  strongly  suggest  that  they  may  be  related  to 
some  of  the  worms. 

285.  General  Characters. 

1.  Body   soft,    unsegmented,    bilaterally   symmetrical    and 
without  segmented  appendages. 

2.  The  organ  of  locomotion  is  a  muscular  thickening  of  the 
body,  called  the  foot,  which  is  variously  modified. 

3.  A  thickened  dorsal  fold  of  the  body  wall,   called  the 
mantle,  is  usually  present.     This  encloses  a  space,  external  to 
the  body,  known  as  the  respiratory  chamber. 

4.  The  mantle  secretes  in  many  cases  a  calcareous  shell,  at 
first  single  and  symmetrical,  but  usually  becoming  either  spiral 
or  separated  into  a  right  and  left  valve. 

5.  The  central  nervous  system  usually  consists  of  three  sets 
of  ganglia:  (i)  the  cerebral  or  "brain,"  above  the  mouth,   (2) 
the  pedal,  in  the  foot  and  connected  with  the  cerebral  by  nerves, 
and  (3)  the  visceral,  also  connected  with  the  brain  by  a  pair  of 
nerves  (Fig.  38). 

6.  Except  in  the  headless  forms  (Acalephs)  a  tooth-bearing 
ribbon,  the  odontophore,  is  found  in  the  mouth. 

286.  General  Survey. — The  more  commonly  known  forms 
are   easily   recognizable   by   the   hard   calcareous   shell   which 


MOLLUSCA  245 

protects  the  soft  unsegmented  body  within.  The  shell  may  be 
in  two  sub-equal  valves,  right  and  left,  or  may  be  in  one  piece, 
in  which  case  it  is  usually  coiled  or  spiral  (Fig.  no).  The 
bivalved  types  are  able  to  open  and  close  the  shell  after  the 
manner  of  a  box,  and  the  soft  parts  are  further  capable  of 
protrusion  from  the  partly  opened  shell.  This  latter  power 
is  much  more  pronounced  in  the  univalved  types  (e.g.,  snail). 
The  fundamental  bilateral  symmetry  is  obscured  in  the  more 
sluggish  forms,  but  is  very  decided  in  such  active  animals  as 
the  squid  and  some  of  the  bivalves. 


FlG.  109.  Shell  of  a  Bivalve  Mollusk,  inner  surface,  ad.a.,  depression  showing  the  attach- 
ment of  the  anterior  adductor  muscle;  ad. p.,  posterior  adductor  muscle;  h,  hinge  with  teeth;  /, 
attachments  of  the  ligaments;  p,  pallial  line,  marking  the  attachment  of  the  mantle  muscles;  5, 
the  pallial  sinus,  marking  the  attachment  of  the  retractor  muscles  of  the  siphon;  u,  umbo  or  beak. 

Questions  on  the  figure. — Which  is  the  dorsal  and  which  the  ventral  portion 
of  the  shell?  Is  this  the  right  or  left  valve?  What  is  the  effect  of  the  contraction 
of  the  adductor  muscles?  What  is  the  value  of  the  teeth  on  the  hinge?  To  what 
point  in  the  shell  of  the  snail  does  the  umbo  correspond? 

One  of  the  most  interesting  points  of  difference  among  the 
members  of  the  group  is  the  degree  of  development  of  the 
"head."  In  the  bivalves  (lamellibranchs)  there  is  a  very 
slight  cephalization,  or  collection  of  special  organs  about  the 
anterior  end.  For  this  reason  they  are  often  called  Acalephs. 
In  the  gasteropods  (snails,  etc.)  and  cephalopods  (squid),  on 


246  ZOOLOGY 

the  other  hand,  the  head  is  well  developed  both  as  to  special 
mouth  parts  and  nervous  organs. 

The  forms  with  shells  are  somewhat  more  limited  in  size 
than  the  cephalopods,  which  furnish  the  largest  representatives 
of  the  phylum,  measuring  in  extreme  cases  20  to  40  feet  in  the 
reach  of  the  arms. 

The  calcareous  shell  insures  abundant  fossil  remains,  repre- 
sentatives being  found  in  various  geologic  formations  from  the 
beginning  of  the  Palaeozoic  era  to  the  present. 

FIG.  no. 


— t 


xut. 


FIG.  no.  Helix.  A,  an  empty  shell  in  section  from  apex  to  base.  Bt  the  relation  of  the 
animal  to  the  shell  when  extended,  a,  apex  of  shell;  an.,  anus;  ap.t  aperture  of  shell;  c,  columella 
or  axis  of  shell;  e,  eyestalk;  /,  foot;  I,  lip  of  shell;  m,  edge  of  mantle,  which  secretes  the  shell;  r.o., 
respiratory  aperture;  5,  suture,  between  the  whorls;  t,  tentacles. 

Questions  on  the  figures. — What  suggestions  of  bilateral  symmetry  are 
shown  by  the  snail?  Where  does  growth  occur  in  the  shell?  What  are  the  func- 
tions of  the  tentacles?  What  is  the  function  of  the  edge  of  the  mantle  called  the 
"collar"  (m)? 

287.  Integument  (skin). — This  consists  of  a  layer  of  epi- 
dermal cells,  covering  a  deeper  dermal  layer  derived  from  the 
mesoderm.  The  former  is  made  up  chiefly  of  the  supporting 
cells  and  the  simple  glandular  cells  which  secrete  mucus,  or 
lime,  or  pigment.  In  many  forms  a  large  portion  of  the  epi- 
thelium in  the  mantle  cavity  (as  the  inner  surface  of  the  mantle 
and  the  covering  of  the  gills  in  Lamellibranchs)  is  ciliate.  The 


MOLLUSCA  247 

dermis  is  a  complex  of  connective  tissue,  muscle  fibres,  pigment 
cells,  etc.  The  mantle  is  a  fold  of  the  skin  which  is  very  char- 
acteristic of  Mollusca.  It  grows  out  from  the  dorsal  wall  of 
the  body  and  encloses  a  space  known  as  the  mantle  cavity. 
It  is  usually  important  in  respiration,  and  contains  the  shell- 
glands. 

288.  Shells  are  formed  in  all  the  classes  of  Mollusca,  al- 
though naked  forms  occur  in  several  of  them.     The  shell  is  a 
true  secretion  or  excretion,  deposited  by  the  epithelial  layer  of 
the  mantle.     It  consists  of  three  layers:   (a)   a  thin  external 
layer  of  organic  material  known  as  conchiolin,  (b)  the  prismatic 
layer,  embracing  the  greater  thickness  of  the  shell  and  made  up 
of  prisms  of  carbonate  of  lime  cemented  by  conchiolin,  and  (c) 
the  nacreous  or  pearly  layer  over  the  inner  surface.     The  edge 
of  the  mantle  secretes  the  first  and  second  layers,  and  they 
usually  show  lines  of  growth  parallel  with  the  edge  of  the  mantle; 
the  pearly  layer  is  deposited  by  the  whole  surface  of  the  mantle. 
The  point  of  attachment  of  the  muscles  presents  a  depression  in 
this  layer  because  the  deposit  has  been  interrupted  (see  pallial 
line  and  muscle  scars,  Fig.  109;  and  in  shell  of  clam). 

In  some  Cephalopods  there  is  an  internal  skeleton  in  part 
secreted  by  the  mantle  (cuttle  bone),  and  in  part  formed  of 
cartilage  (the  brain  case). 

289.  The  muscular  system  is  made  up  of  unstriped  muscle 
fibres,  which  usually  occur  in  more  or  less  prominent  bands  or 
muscles.     These  may  be  classified  as  follows:  (i)  shell  or  skeletal 
muscles,  which  embrace  (a)  adductors,  those  which  draw  the 
valves  together  (lamellibranchs),  (b)  retractors,  which  withdraw 
the  whole  or  special  portions  of  the  animal  into  the  shell  (lamel- 
libranchs and  gasteropods) ,  (c)  protractors,  or  extensors,  which 
enable  the  animal  partly  to  extend  itself;  (2)  pallial  (mantle) 
muscles,  best  developed  in  cephalopods;  (3)  the  foot,  which  is  a 
mass  of  muscle  and  is  one  of  the  most  characteristic  of  the 
molluscan  organs;  and  (4)  minor  muscles  controlling  the  radula 
or  tongue,  the  other  mouth  parts,  and  the  like. 

Locomotion  in  the  group  is  accomplished  chiefly  by  the  foot, 
in  its  various  modifications,  or  by  rhythmic  opening  and  shut- 


248  ZOOLOGY 

ting  of  the  valves.  The  squid  has  a  fin-like  extension  of  the 
integument  which  is  an  efficient  organ  of  forward  motion.  The 
siphon  of  the  same  animal  is  regarded  as  a  modification  of  a  part 
of  the  foot.  The  tentacles  about  the  mouth  are  also  looked 
upon  as  arising  from  the  anterior  part  of  the  foot,  hence  the 
name  Cephalopoda,  which  means  "head-footed." 

2 go.  Digestive  Organs. — Mouth  and  anus  both  occur,  and 
are  usually  widely  separated.  In  the  coiled  forms  (as  the  snail), 
however,  by  the  looping  of  the  digestive  tract  they  are  brought 
close  together.  In  all  except  the  group  of  headless  mollusks 
(lamellibranchs)  the  mouth  is  supplied  with  a  radula,  or  tooth- 
bearing  tongue.  This  lies  in  the  floor  of  the  mouth  and,  as  it  is 
worn  away  in  front,  is  renewed  from  behind  in  the  radula  sac 
(Fig.  in).  It  rasps  small  particles  from  solids  and  conveys 


FIG.  in.  Diagram  of  mouth  of  snail,  showing  the  lingual  ribbon  (radula).  br,  brain;  c,  buccal 
cavity;  co.,  ccelom;  g,  gullet;  j,  jaw,  against  which  the  radula  works;  m,  mouth;  r.t  radula;  r.s., 
radula  sac,  in  which  the  radula  is  renewed  as  it  is  worn  away  in  front. 

Questions  on  the  figure. — What  parts  go  to  make  up  the  "odontophore?" 
How  do  the  parts  act  in  biting? 

them  backward  into  the  esophagus.  In  the  gasteropods  there 
is  a  plate  in  the  upper  jaw  against  which  this  organ  works.  In 
the  cephalopods  beak-like  jaws  occur  suited  to  their  carnivorous 
habit.  The  mouth  is  followed  by  a  gullet,  which  may  com- 
municate at  once  with  the  stomach  (lamellibranchs),  or  may 
expand  into  a  crop  (gasteropods  and  cephalopods) .  The  stomach 
is  well  marked  and  opens  into  the  intestine  which  is  usually  long 
enough  to  make  one  or  more  coils  in  the  body  mass.  It  may 
open  externally  (gasteropods)  or  in  the  mantle  chamber  (cepha- 
lopods and  lamellibranchs).  Salivary  glands  pour  their  secre- 


MOLLUSCA 


249 


tion  into  the  mouth  cavity  or  into  the  gullet,  and  the  so-called 
liver  connects  with  the  stomach  or  intestine. 

291.  Respiration. — The  oxygen  may  be  derived  from  the 
water  (lamellibranchs,  cephalopods,  and  some  gasteropods) 
or  from  the  air  (pulmonate  gasteropods).  In  the  latter  a 
pulmonary  chamber  is  formed  by  the  mantle.  Blood  is  richly 
supplied  to  the  walls  of  this  sac  and  is  there  aerated  after  the 
manner  of  lungs.  In  the  water-breathing  forms  the  gills  are 

FIG.  112. 


—  m 


FIG.  112.  Diagram  showing  the  heart  and  general  course  of  the  circulation  in  the  Lamelli- 
branchs. Only  a  short  section  is  shown,  a,  auricle  (right),  with  slit  to  ventricle;  b,  the  body 
(region  of  spaces,  lacunae,  capillaries);  g,  the  region  of  the  gills,  with  capillaries;  k,  kidneys,  with 
their  capillaries;  m,  the  mantle  and  capillaries;  v,  the  ventricle  from  which  arteries  pass  forward  and 
backward;  v.c.,  "vena  cava,"  in  which  the  blood  collects  on  returning  from  the  tissues  of  the  body. 

Questions  on  the  figure. — Follow  by  the  arrows  and  letters  the  general  course  of 
the  blood  flow.  How  many  sets  of  capillaries  are  passed  by  the  blood  which  goes 
to  the  mantle?  By  that  which  goes  to  the  system,  before  returning  to  the  heart? 
What  changes  take  place  in  the  blood  in  the  capillaries  of  the  various  regions? 

variously  constructed.  Lamellibranchs  possess  a  pair  of  "gill- 
plates"  hanging  in  the  mantle  cavity  on  either  side  the  body. 
These  are  made  up  of  an  immense  number  of  ciliated  tubular 
filaments  which  intercommunicate  in  a  complicated  lattice- 
work. To  the  naked  eye  they  appear  as  thin  sheets  with  stria- 


25° 


ZOOLOGY 


tions  passing  from  the  dorsal  to  the  ventral  margin  (see  dissec- 
tion of  clam).  The  walls  of  the  gills  contain  blood  vessels,  and 
the  water,  assisted  by  the  action  of  the  cilia,  circulates  over  and 
through  the  gills.  In  the  cephalopods  and  aquatic  gasteropods 
the  gills  occur  as  tufts  of  filaments,  which  may  or  may  not  be 
covered  by  the  mantle.  In  addition  to  these  special  organs  the 
mantle  and  the  soft  body  surface  assist  in  respiration.  (For 
figures  of  the  gill  structure  in  the  clam  see  Parker  and  Has  well's 
Text-book  of  Zoology,  Vol.  I,  Fig.  529.) 

292.  Circulation. — There  is  usually  a  well-developed  circu- 
lation of  the  blood,  but  a  portion  of  it  occurs  through  irregular 
spaces  devoid  of  proper  walls.  The  organs  consist  of  a  contrac- 
tile heart  usually  with  undivided  ventricle  and  a  single  auricle 


tissues 


tissues 


PIG.  113.     Diagram  showing  the  general  course  of  the  circulation  in  mollusks.     Compare  with 
Fig.  112,  which  shows  the  organs  more  nearly  in  their  relative  position. 

Question  on  the  figure. — Why  does  the  blood  which  passes  to  the  mantle  not 
need  to  pass  the  gills  before  returning  to  the  heart? 

(gasteropods),  or  one  pair  of  auricles  (lamellibranchs,  squid), 
or  two  pairs  (Nautilus).  Definite  arteries  pass  both  forward 
and  backward  from  the  ventricle.  The  blood  passes  from  the 
ventricle  to  the  tissues  of  the  body,  whence  it  gathers  into 
venous  spaces  and  passes  into  the  kidneys  and  the  gills  by  way  of 
a  principal  vein.  From  the  gills  it  finds  its  way  to  the  auricles. 
In  lamellibranchs  the  blood  which  goes  from  the  ventricle  to 
the  mantle  is  aerated  and  returns  directly  to  the  auricle.  In 
some  Cephalopods  there  are  branchial  hearts  near  the  gills  to 


MOLLUSCA  251 

assist  the  return  of  the  blood  to  the  heart.  The  accompanying 
diagrams  (Figs.  112,  113)  will  help  the  student  follow  the  main 
facts  of  the  circulation.  In  lamellibranchs  the  ventricle  often 
surrounds  the  intestine.  The  corpuscles  are  colorless  and 
amoeboid.  The  plasma,  however,  quite  commonly  contains  a 
bluish  pigment  (hcemocyanin)  which  assists  respiration  in  much 
the  same  way  as  the  haemoglobin  of  the  vertebrates. 

293.  Excretory  Organs. — In  mollusks  the  excretory  organs 
consist,  when  reduced  to  the  simplest  terms,  of  one  or  more 
nephridia  which  communicate  interiorly  with  the  pericardium 
or  principal  ccelomic  space,  and  with  the  exterior  by  way  of  a 
tubular   ureter.     The   kidney   portion    of    the   tube   is    much 
modified,  has  glandular  walls  and  is  well  supplied  with  blood 
vessels.     It  lies  in   the  immediate   region   of  the  pericardial 
chamber  in  most  cases. 

294.  Nervous   System. — The  nervous  system  of  mollusks 
is  usually  made  up  of  at  least  three  pairs  of  ganglia:     (a)  the 
"brain"  or  cerebral  ganglia  dorsal  to  the  mouth  and  varying 
in  size  according  to  the  degree  of  development  of  the  head; 
(6)  connected  with  the  brain  by  a  pair  of  connectives  are  the 
pedal  ganglia  lying  ventral  to  the  mouth  and  innervating  the 
foot;  (c)  the  pi  euro- visceral  ganglia  variously  situated  in  the 
different  groups  and  connected  with  the  brain  or  both  with  the 
brain  and  the  pedal  ganglia.     From  it  nerves  pass  to  the  mantle, 
and  to  the  posterior  organs.     In  gasteropods  and  cephalopods 
all  these  ganglia  are  much  closer  together  and  are  collected 
about   the   mouth.     Still   other   ganglia   are   often   associated 
with  them.     The  student  should  notice  how  this  collection  of 
nervous    matter    accompanies    the    development    of    "head" 
organs  in  the  better  developed  members  of  the  phylum. 

295.  The   Organs  of   Special   Sense. — As  usual,   scattered 
sensory  cells  are  situated  in  the  exposed  epithelial   surfaces. 
These  .give  rise  to  a  diffuse  sensitiveness  to  tactile  and  chemi- 
cal stimuli.     The  edges  of  the  mantle  and  the  tentacles  are 
especially    sensitive.     Patches    of    sensory    cells — osphradia— 
are  often  found  near  the  bases  of  the  gills,  which  probably 


252 


ZOOLOGY 


have  a  value  in  testing  the  character  of  the  water  flowing  over 
them.  Still  other  patches  occur  about  the  lips.  Otocysts  (see 
§m),  or  statocysts,  occur  in  all  the  groups.  Eyes  are  usually 
found  and  are  of  various  degrees  of  complexity.  They  are 
simplest  in  the  lamellibranchs  (Fig.  43),  and  when  found  at  all 
in  this  group  may  occur  in  great  numbers  along  the  mantle. 

FIG.  114. 


FIG.  114.  Diagram  of  a  dissection  of  the  reproductive  organs  of  a  snail,  a.g.,  albumen  gland; 
c.d.,  common  or  hermaphrodite  duct;  e.g.,  hermaphrodite  gland;  d.s.,  dart  sac;/,  flagellum;  g,  genital 
aperture;  m.g.,  mucous  glands;  o,  oviduct;  p,  penis;  r.s.,  receptaculum  seminis;  v.d.,  vas  deferens. 
The  slit  from  the  genital  aperture  into  the  oviduct  and  penis  shows  the  openings  of  the  dart  sac, 
mucous  glands,  and  the  receptaculum  seminis.  (After  Pelseneer.) 

Questions  on  the  figure. — By  a  careful  study  of  the  figure  and  the  text,  de- 
termine the  functions  of  the  various  parts  of  the  system.  Does  self-fertilization 
occur  in  a  form  like  this?  Evidences. 

In  the  gasteropods  the  eyes  are  borne  on  the  ends  of  tentacles 
and  are  frequently  destroyed  by  accidents.  The  animals  have 
the  power  of  regenerating  the  tentacle, — eye  and  all.  This 
manifestly  is  a  very  useful  adaptation.  The  eyes  of  cephalopods 
are  the  most  perfect  single  eyes  found  among  the  invertebrates. 


MOLLUSC A  253 

Though  originating  in  a  different  way,  these  are  strikingly  like 
the  vertebrate  eye. 

The  forms  with  shells  are  not  so  responsive  to  stimuli  as 
free  types  such  as  cephalopods,  but  even  the  snails,  and  in  less 
degree  the  clams,  are  sensitive  to  light,  contact,  gravity, 
chemical  states  of  the  air  and  water  that  would  in  us  arouse  the 
sense  of  taste  or  smell.  The  most  characteristic  reaction  of  the 
lower  forms  is  to  withdraw  into  the  shell  or  to  close  it.  Gravity 
and  light  and  odor  also  direct  their  motions. 

296.  Library  Reference. — Make  a  report  on  the  position  and  general  structure 
of  the  eyes  in  gasteropods,  cephalopods  and  lamellibranchs. 

297.  Reproduction  and  the  Genital  Organs. — Reproduction 
is  always  sexual.     In  some  of  the  lamellibranchs  (e.g.,  oyster) 
and  many  of  the  simpler  gasteropods,  including  the  land  snails, 
the  individuals  are  hermaphrodite.     The  sexes  are  separate  in 
the  cephalopods  and  in  most  of  the  lamellibranchs  and  gastero- 
pods.     The    organs    are   more   complicated   among    the   her- 
maphrodite gasteropods  than  elsewhere  in  the  group  (see  diagram, 
reproductive  organs  of  snail,  Fig.  114).     The  sexual  glands  are 
usually  situated  in  the  visceral  mass  among  the  coils  of  the 
intestine.     The  ducts  ordinarily  open  into  the  mantle  cavity, 
where  fertilization  may  occur.     The  eggs  after  fertilization  are 
often,  either  singly  or  in  masses,  surrounded  by  a  gelatinous 
secretion  (as  in  the  snail)  which  serves  as  a  protection  from 
drouth  and  as  a  means  of  attachment.     In  lamellibranchs  the 
young  are  not  infrequently  retained  in  the  mantle  or  respiratory 
chamber  until  partly  developed. 

298.  Development. — Segmentation  is  total  (lamellibranchs 
and  gasteropods)  or  partial  anddiscoidal  (dibranch  cephalopods). 
It  is  usually  unequal  in  the  lamellibranchs  and  gasteropods, 
but  in  some  of  the  latter  it  is  equal  during  the  first  two  divisions, 
producing  four  equal  blastomeres.     Each  of  these  divides  into 
a  large  and  a  small  cell — macromere  and  micromere.     Still  other 
micromeres  are   formed    at   the  expense  of  the  macromeres, 
and  these  by  continued  division  form  a  cap  of  ectodermal  cells 
(Fig.  115).     From  the  macromeres  arise  ultimately  the  ento- 
derm  and  mesoderm.     The  gastrula  may  be  formed  either  by 


254 


ZOOLOGY 


in  vagi  nation  of  the  large  cells  or  by  the  overgrowth  of  the 
micromeres,  depending  on  the  size  of  the  segmentation  cavity 
and  of  the  entodermal  cells.  In  the  cephalopods,  owing  to  the 
large  supply  of  food  substance  in  the  ovum,  cleavage  is  confined 
to  a  small  disc  at  the  active  pole.  From  this  point  where  the 
embryo  is  destined  to  be  developed,  a  sheet  of  cells  gradually 
extends  itself  by  growth  around  the  yolk.  Thus  a  yolk-sac  is 
formed  by  means  of  which  the  food  is  used  in  the  further  de- 
velopment of  the  embryo.  By  the  time  the  embryo  is  hatched 
the  yolk  is  exhausted.  Although  the  yolk  does  not  segment  we 
see  that  it  serves  its  purpose  in  the  development  of  the  embryo. 

FIG.  115. 


mi. 


mes.- — 


FlG.  115.  Diagram  of  early  segmentation  stages  in  a  Gasteropod.  A,  2-celled  stage;  B,  4 
celled;  C,  8-celled;  Dt  later  stage,  in  section,  ect.,  ectoderm  cells  (micromeres);  ent.,  entoderm  cells, 
macromeres;  mes.,  mesoblasts,  early  put  aside, — before  gastrulation — to  form  the  mesoderm;  mi., 
micromeres;  ma.,  macromeres. 

Questions  on  the  figures. — What  causes  are  assigned  for  the  differece  in  the 
size  of  the  cells  in  the  8-celled  stage?  In  what  other  ways  is  mesoderm  formed 
in  the  metazoa?  Which  cells  seem  to  divide  more  rapidly,  the  micromeres  or  the 
macromeres?  Compare  with  Annelid,  Fig.  105. 

The  later  development  is  typically  indirect,  i.e.,  with  a  metamor- 
phosis, though  many  (as  the  cephalopods)  develop  directly  into 
the  adult  form.  A  larval  stage  (trocko sphere)  occurs,  suggesting 
the  larvae  of  the  Polychaeta.  This  is  followed  by  another  stage 
(veliger)  which  is  more  characteristic  of  the  Mollusks. 

299.  Library  Exercises. — Students  may  well  supplement  the  text  by  making 
short  reports  on  the  following  topics:  the  early  segmentation  of  lamellibranchs 
and  gasteropods;  of  the  cephalopods;  the  veliger  of  mollusks;  the  formation  of  the 


MOLLUSCA  255 

organs  in  cephalopods;  development  in  the  clam  or  mussel.     Illustrations  should 
be  found  in  the  advanced  text-books  and  presented  to  the  class. 

300.  Ecology. — The  bivalves  are  sedentary  or  sluggish  in 
their  manner  of  life;  the  motion  of  most  of  the  gasteropods 
is  slow  and  difficult.  In  conformity  with  their  limited  powers 
of  locomotion,  they  are  scavengers,  feeding  on  the  debris  and 
the  small  animals  and  plants  brought  to  them  by  the  water 
currents  (oysters,  mussels,  etc.),  or  are  largely  herbivorous 
(many  snails).  A  very  few  are  parasitic.  The  cephalopods 
are  much  more  active  and  are  carnivorous.  For  the  most  part 
the  sluggish  forms  are  well  protected  by  the  shells,  neverthe- 
less they  furnish  food  for  many  diverse  sorts  of  animals.  Some 
of  their  enemies  are  internal  parasites,  others  bore  through  the 
shells  and  thus  gain  access  to  vital  parts. 

The  animal  within  may  thwart  this  attack  of  its  enemies  by 
the  continued  secretion  of  mother-of-pearl  on  the  inner  sur- 
face at  the  threatened  point.  Some  animals  crush  the  shells, 
or  swallow  the  mollusks,  shell  and  all.  Starfishes,  as  we  have 
seen,  are  especially  troublesome  to  the  oyster  beds. 

Many  of  the  bivalves  are  capable  of  still  further  protection 
because  of  their  elongated  siphons  which  enable  them  to 
burrow  deeply  in  the  mud  or  sand,  the  food  being  carried  in 
through  the  siphons  by  the  water  currents  (Fig.  116).  Several 
species  of  marine  bivalves  have  the  power  of  boring  into  wood 
or  even  stone.  This  serves  as  a  protection  to  them,  but  often 
results  in  the  complete  destruction  of  piles  and  other  structures 
placed  in  the  ocean  by  man. 

Many  of  the  mollusks  seem  more  or  less  gregarious,  as  is 
illustrated  by  beds  of  clams  and  oysters,  the  schools  of  squid, 
etc. 

Notwithstanding  the  low  organization  and  sluggishness  of 
a  large  portion  of  the  branch  Mollusca,  we  are  compelled  to 
consider  that  it  has  been  a  very  successful  group  in  that  it 
has  held  its  place  with  practically  equal  numbers  through  the 
geological  ages,  and  has  succeeded  in  adapting  itself  to  the 
changes  of  those  ages.  Of  no  less  interest  is  the  additional 
fact  that  there  is  scarcely  a  nook  into  which  they  have  not 
penetrated,  except  where  continuous  drouth  prevails.  On  the 


256 


ZOOLOGY 


other  hand,  it  is  among  the  more  active  types — the  cephalo- 
pods — that  the  ancient  geological  forms  have  least  success- 
fully adapted  themselves  to  modern  conditions.  The  cephalo- 
pods  appear  much  less  numerous  and  varied  now  than  in  earlier 
geological  time,  although  the  types  which  have  persisted  are 
the  most  perfect  and  active  ones  that  have  appeared. 

301,  Relation  to  Human  Interests. — This  is  the  first  phylum 
that  we  have  studied  that  furnishes  any  food,  worthy  of  mention, 

FIG.  1 1 6. 


PIG.  116. 


Mya  arenaria,  a  burrowing  clam.     The  siphon  is  represented  as  fully  extended, 
quickly  retracted  when  the  animal  is  disturbed.     (After  Kingsley.) 


This  is 


Questions  on  the  figure. — What  is  the  function  of  the  much  elongated  siphons? 
Which  is  the  anterior  end  of  the  animal?  Which  the  dorsal  side?  What  would 
seem  to  be  the  chief  function  of  the  foot  in  this  case? 

to  man.     It  is  estimated  that  $65,000,000  annually  are  derived 
from  our  oyster  fisheries.     There  are  several  species  of  clams 


MOLLUSCA  257 

and  mussels  that  are  used  for  food.  Less  frequently  snails  and 
squid  are  eaten. 

Pearl  buttons,  knife  handles,  and  other  objects  are  made 
from  the  "mother-of-pearl"  shells  of  mollusks.  Pearls,  one  of 
the  most  prized  ornaments,  are  produced  by  a  number  of  species, 
notably  fresh-water  clams  and  the  marine  pearl  oyster. 

Squid  produce  cuttle  bone  and  sepia,  and  are  used  for  codfish 
bait. 

The  oyster  industry  and  the  clam-shell  industry  have  become 
so  extensive  that  they  have  ceased  to  be  merely  "fisheries." 
Promiscuous  fishing,  if  profitable,  always  results  in  threatening 
the  whole  industry.  The  next  step  is  to  limit  fishing  or  devise 
means  of  encouraging  the  growth  of  the  organisms,  or  both. 
This  has  been  done  for  the  oyster  and  is  beginning  for  the  clam. 
The  steps  involved  are  to  favor  the  young  by  artificial  means,  to 
give  them  suitable  places  to  settle,  to  furnish  them  with  food, 
and  to  protect  them  from  enemies  and  unfavorable  environment. 
This  is  a  form  of  "farming,"  rather  than  fishing.  Coupled  with 
legal  limitations  upon  fishing  such  measures  make  for  steadiness 
of  supply,  and  should  be  encouraged  in  every  way  possible. 

302.  Classification. — The  following  are  the  principal  classes: 
Class  I.  Pelecypoda    (hatchet-footed)   or  Lamellibranchiata;  (Mussels,  Oysters, 
etc.). — Lamellibranchs  are  mollusks  'in   which  the   fundamental   bilateral   sym- 

FIG.  117. 


FIG.   117.     Ensis  amcricanus,  the  razor  clam.     From  Verrill,  after  Gould. 

Questions  on  the  figure. — Where  is  the  hinge,  the  umbo,  etc.  ?     Trace  the  lines 
of  growth  and  compare  with  other  figures  of  bivalves. 

metry  is  shown  in  the  right  and  left  valves  of  the  shell  secreted  by  a  bilobed 
mantle,  and  in  some  of  the  internal  organs.  There  may  be  one  or  two  adductor 
muscles.  The  head  is  undeveloped.  The  ventral  body  region  is  differentiated 
into  a  muscular  foot,  shaped  like  a  plow-share.  The  gills  are  in  sheets  (see 
§291)  usually  two  on  either  side,  and  are  suspended  in  the  mantle  cavity.  Paired 
labial  palps  occur  about  the  otherwise  unspecialized  mouth.  The  three  pairs  of 
ganglia — the  cerebro-pleural,  the  pedal,  and  the  visceral, — are  usually  well  sepa- 
17 


258 


ZOOLOGY 


rated.  The  heart  consists  of  two  auricles  and  one  ventricle  surrounded  by  a 
pericardial  space,  which  is  a  portion  of  the  body  cavity  and  communicates  with 
the  exterior  by  a  pair  of  nephridial  tubes.  The  reproductive  organs  are  simple; 
the  sexes  are  ordinarily  separate.  Development  by  a  metamorphosis. 

[The  primary  subdivisions  of  the  group  may  be  made  on  the  basis  either  of  the 
gill  structure,  the  adductor  muscles,  or  the  presence  or  absence  of  the  siphon.] 

Order  I.  Isomya:  Two  adductor  muscles  which  are  essentially  equal.. 

(a)  Siphon  well  developed,  retractile;  pallial  line  (Fig.  109)  with  a  sinus. 
Here  occurs  Mya  arenaria,  the  common  clam  of  the  Atlantic  coast.  Great  heaps 
of  shells  of  this  clam  show  that  it  was  much  used  by  the  Indian  tribes  as  food. 
In  New  England  the  clam  fisheries  are  of  very  considerable  importance.  Mya 
burrows  in  the  mud,  using  its  long  siphon  to  keep  it  in  connection  with  the  water 
from  which  it  gets  its  food.  Of  somewhat  similar  habits  is  the  razor-shell  clam. 

FIG.  i i 8. 


FIG.  118.     Mytilus  edulis,  a  Mussel.     From  Binney's  Gould. 

Questions  on  the  figure. — Identify  the  umbo.  What  are  your  evidences  that 
it  is  the  umbo?  Compare  the  lines  on  the  shell  with  those  in  Fig.  117.  What 
is  the  significance  of  the  specific  name  (edulis}  ?  What  are  the  habits  of  the  species  ? 


(Fig.  117).  The  "borer"  (Pholas)  and  the  "ship-worm"  (Teredo}  belong  to  this 
group  and  possess  the  power  of  boring  into  wood  or  stone  and  are  thus  of  much 
damage  to  submerged  structures  in  waters  where  they  abound. 

(b)  Siphon  usually  present  but  not  highly  developed;  no  pallial  sinus.  In  this 
group  are  embraced  the  more  abundant  fresh-water  mussels  (Unio,  Anodonta, 
Cyclas),  and  the  cockles  (Cardium}  of  the  ocean.  The  Unionidae  are  very  widely 
distributed  and  very  common  in  our  own  fresh  waters.  They  are  not  much  used 
for  food  at  present,  though  the  Indians  used  them,  probably  in  times  of  scarcity  of 
other  food.  Their  shells  are  widely  employed  in  the  making  of  buttons,  knife 
handles  and  the  like,  and  pearls  of  value  are  not  of  infrequent  occurrence.  These 
are  merely  the  mother-of-pearl,  which  ordinarily  lines  the  shell,  secreted  about 


MOLLUSCA 


259 


some  infected  or  irritated  point  on  the  epidermis  of  the  mantle.     Great  quantities 
of  these  pearls  are  sometimes  found  in  the  graves  of  the  mound  builders. 

In  some  of  the  fresh-water  clams  there  is  an  interesting  temporary  parasitism 
during  the  early  development.  When  the  young,  called  glochidia,  escape  from  the 
gills  of  the  mother,  they  must  become  attached  to  the  skin  or  gills  of  a  fish. 
They  may  become  deeply  imbedded  and  be  nourished  in  the  tissues  of  the  host  for 
several  weeks.  During  this  time  they  pass  through  their  metamorphosis  and 
develop  the  adult  organs.  This  habit  brings  about  a  better  distribution  of  the 
clams  than  would  otherwise  happen,  since  it  has  been  shown  that  adult  clams  do 
not  voluntarily  migrate  widely. 

FIG.  119. 


PIG.  119.     Pecten  irradians, — a  Scallop.     From  Binney's  Gould. 

Questions  on  the  figure. — Is  this  an  external  or  internal  view  of  the  shell? 
Where  is  the  umbo?  What  is  peculiar  about  the  hinge  in  this  case?  What  is  the 
significance  of  the  lines  nearly  concentric  with  the  margin?  Of  the  radial  lines? 

Order  2.  Heteromya:  Two  adductor  muscles,  the  anterior  much  reduced; 
siphon  usually  wanting.  Here  are  included  the  horse-mussel  (Modiola)  and 
Mytilus,  edible  mussels  which  occur  in  clusters  just  below  low  tide  mark;  also  the 
pearl-oyster,  from  which  the  best  pearls  are  taken.  The  last  mentioned  form  is 
not  found  on  our  own  coasts. 

Order  3.  Monomya:  One  adductor  muscle  (posterior);  no  siphon.  The  genus 
Ostrea  (oyster)  and  the  genus  Pecten  (scallop)  are  the  most  interesting  and  impor- 
tant representatives  of  this  order.  The  species  of  Ostrea  differ  much  in  size  in 
different  regions.  The  largest  living  species  is  a  Japanese  form  which  is  known  to 
reach  a  length  of  two  to  three  feet.  The  oyster  is  hermaphrodite.  The  young 
after  a  short  free  life,  become  attached  by  one  of  the  valves.  The  oyster  constitutes 
a  larger  element  in  the  food  supply  of  man  than  any  other  invertebrate.  The 
scallops  are  not  attached,  and  swim  by  a  rapid  opening  and  closing  of  their  valves. 


260 


ZOOLOGY 


Class  II.  Gasteropoda  (belly-footed;  Snails,  Slugs,  Whelks,  and  Periwinkles).— 
Gasteropods  are  mollusks  with  unsymmetrical,  univalved,  usually  spiral  shells 
(occasionally  lacking  the  shell  altogether).  The  head  and  foot  ordinarily  preserve 
the  bilateral  symmetry,  but  the  other  organs  lose  their  symmetry  both  from  the 
spiral  form  of  the  shell  and  from  a  twisting  which  many  of  the  forms  undergo  by 
which  the  nervous  system  and  certain  other  visceral  organs  lose  their  original  right 
and  left  relations.  The  head  region  is  well  developed,  having  tentacles,  eyes,  and 
a  mouth  with  a  tooth-bearing  radula.  Gills  in  the  mantle  cavity  two,  one,  or 
none;  in  the  air-breathing  forms  there  may  be  merely  a  pulmonary  sac.  The  sexes 
are  separate  (Streptoneura}  or  united  in  one  individual  (land  snails).  Development 
is  mostly  indirect. 


PIG.  120. 


FIG.  121. 


PIG.  120.     Acmcea  lestudinalis  (Limpet),  from  Binney's  Gould.     Upper  figure  lateral  view;  lower 

figure,  dorsal  view. 

Questions  on  the  figure. — How  do  the  Limpets  differ  from  the  majority  of  the 
snails?  What  is  the  appropriateness  of  the  specific  name  (testudinalis)? 

FIG.  I2i.     Helix  albolabris,  a  pulmonate  Gasteropod.     From  Binney's  Gould. 

Questions  on  the  figure. — What  is  the  significance  of  Helix?  Of  albolabris? 
Identify  the  parts  of  the  shell.  Is  it  a  right  or  left  spiral?  What  do  you  mean  by 
your  answer? 


Subclass  I.  Streptoneura. — Gasteropods  in  which  the  nerve  loop  made  by  the 
visceral  commissures,  is  twisted  in  development  into  the  form  of  the  figure  8 ;  the 
other  visceral  organs  are  twisted  so  that  right  and  left  are  interchanged.  Only  one 
pair  of  tentacles  on  the  head.  Sexes  separate.  Gills  usually  in  front  of  the 
heart. 

One  of  the  common  representatives  of  this  group  is  Littorina,  the  common 
periwinkle  of  the  seashore.  Many_pther  types  of  almost  infinite  variety  of  form, 
size,  and  color  inhabit  the  ocean,  their  shells  often  being  washed  ashore  by  the 
waves;  such  are  the  cowries,  the  whelks,  the  cone-shells,  etc.  Here  belong  the 
uncoiled  Limpet  and  the  slightly  coiled  Crepidula  or  boat-shell. 

Subclass  II.  Euthyneura  (Land  Snails  and  many  naked  Mollusks). — Gastero- 


MOLLUSCA 


26l 


pods  in  which  the  nerve  loop  is  not  twisted.  The  head  usually  bears  two  pairs  of 
tentacles.  The  sexes  are  united  in  the  same  individual.  The  most  important 
of  these  are  the  Pulmonata  or  air  breathing  Gasteropods,  some  of  which  are 
terrestrial  and  others  aquatic.  Of  the  terrestrial  snails  the  genus  Helix  (Fig.  121) 
is  the  most  widely  distributed  and  interesting.  Its  variability  is  such  that  between 

FIG.  122. 


FIG.  122.     Limax  flavus,  a  Slug.     From  Binney's  Gould. 

Questions  on  the  figure. — How  do  the  slugs  differ  from  the  other  Gasteropods? 
In  what  external  respects  do  they  appear  similar  to  them?  Compare  all  the 
figures  of  slugs  you  may  be  able  to  find. 


FIG.  123. 


e' 


FIG.  123.  Pearly  Nautilus.  From  Nicholson,  e,  eye;  h,  hood,  a  muscular  portion  of  the  foot 
which  protects  the  softer  parts;  s,  siphon;  se,  septa,  separating  the  successive  chambers  of  the  shell; 
sp,  siphuncle;  t,  tentacles. 

Questions  on  the  figure. — How  does  this  shell  compare  with  those  of  the 
Gasteropods?  What  is  considered  to  be  the  homology  of  the  tentacles  or  arms  in 
Cephalopods?  What  is  the  siphuncle?  What  is  the  character  of  the  eye  in 
Nautilus? 


three  and  four  thousand  species  have  been  described.  Limax  (Fig.  122)  is  a  pul- 
monate  form  in  which  the  shell  is  practically  wanting.  It  is  especially  destructive 
to  certain  types  of  plants  as  it  is  a  voracious  vegetable  feeder.  The  aquatic  pul- 
monates  are  represented  by  the  "pond-snail"  (Limncea),  and  by  Planorbis,  a  snail 
whose  coils  are  in  one  plane,  presenting  a  helix  rather  than  a  spiral. 


262  ZOOLOGY 

Class  III.  Cephalopoda  (head-footed;  Squid,  Devil-fish}. — The  cephalopods 
are  bilaterally  symmetrical  mollusks  with  a  well-developed  head  in  which  the  front 
part  of  the  foot  surrounds  the  well-armed  mouth  as  a  series  of  lobes  or  tentacles. 
The  head  protrudes  permanently  from  the  mantle  cavity,  leaving  the  mantle 
surrounding  the  posterior  part  of  the  body.  The  posterior  lobe  of  the  foot  forms 
a  siphon,  communicating  with  the  mantle  cavity.  Into  this  cavity  the  nephridia, 
the  anus,  and  the  reproductive  glands  open,  and  in  it  the  gills  lie.  The  shell  may 
be  present  and  external  (Nautilus},  internal  and  slightly  developed  (Squid),  or 
wanting  (Octopus).  An  internal  cartilaginous  skeleton  protects  the  brain.  The 
ccelom  is  well  developed.  The  ganglia  of  the  nervous  system  are  massed  in  the 
head  region.  The  sexes  are  separate  and  the  development  direct.  The  Cephalo- 
poda are  to  be  looked  upon  as  the  most  highly  developed  of  the  Mollusca.  They 
are  little  in  evidence  now,  however,  as  compared  with  earlier  times. 

FIG.  124. 


FIG.  124.     The  Devil-fish  (Octopus).     From  Cooke,  after  Merculiano.     A,  at  rest;   B.  swimming. 
a,  arms,  with  suckers  on  the  inner  aspect;  «,  eye;  s,  siphon  or  funnel. 

Questions  on  the  figure. — Which  is  the  anterior  end  of  the  animal?  What 
is  the  position  of  the  mouth?  What  is  the  function  of  the  siphon?  Of  what 
structure  is  it  a  part? 

Subclass  I.  Tetrabranchiata. — Cephalopoda  in  which  the  front  segment  of  the 
foot  is  divided  into  lobes  bearing  numerous  tentacles,  without  suckers.  Shells 
external  and  chambered  (and  in  Nautilus,  the  only  living  genus,  coiled).  Two 
pairs  of  auricles;  two  pairs  of  gills;  two  pairs  of  nephridia. 

This  group  is  important  for  its  extinct  rather  than  for  its  living  representatives. 
The  pearly  or  chambered  nautilus  (Fig.  123)  found  in  the  Pacific  and  Indian 
Oceans,  is  the  only  important  living  species.  The  Nautilus  appears  to  be  the  only 
remaining  representative  of  the  once  numerous  coiled  forms  and  more  remotely 
still  of  the  Orthoceratites,  the  rulers  of  the  Palaeozoic  seas  (see  Geology). 

Subclass  II.  Dibranchiata. — Cephalopods  in  which  a  circlet  of  8  to  10  arms 
surround  the  mouth.  These  bear  sucking  discs.  Shell  internal  and  rudimentary 


MOLLUSCA 


263 


or  absent.  One  pair  of  gills,  one  pair  of  nephridia,  and  one  pair  of  auricles.  An 
ink  gland  is  present. 

Order  i,  Decapoda,  embraces  the  cuttle-fish  and  squid. 

Order  2,  Octopoda,  embraces  the  devil-fishes  (Fig.  124)  and  the  paper  nautilus 
(Fig.  125). 

FIG.  125. 


FIG.  125.     The  Paper  Nautilus  (Argonauta  argo).     From  Cooke,  after  Lacaze-Duthiers.     «,  eye; 
ra,  mouth;/,  siphon;  sh,  shell;  /,  tentacles. 

Questions  on  the  figure. — In  what  way  does  the  siphon  serve  in  locomotion? 
In  which  direction  will  the  animal  move  by  means  of  the  siphon?  How  does  the 
shell  of  Argonauta  differ  from  that  of  Nautilus? 

303.  Supplementary  Studies  for  Library,  Laboratory,  and 
Field. 

1.  Compare  the  clam,  snail,  and  squid  with  regard  to  the 
following  particulars,  putting  the  results  in  a  tabular  form : 

(a)  Degree  of  development  of  the  head. 

(b)  Shell,  development  and  method  of  using,  in  each. 

(c)  Mantle;  extent,  form  and  modifications:  mantle  cavity. 

(d)  Foot;  parts,  differentiation,  and  uses. 

(e)  Respiration ;  how  accomplished  ? 

(/)    Sense  organs ;  position,  character,  and  degree  of  develop- 
ment. 

(g)  Locomotion ;  how  effected  ? 
(ti)  Protection;  special  devices. 

2.  Can  you  find  any  indication  among  the  mollusks  of  a 
relation  between  the  degree  of  development  of  the  sense  organs 


264  ZOOLOGY 

and  the  activity  shown  by  the  animals  ?     Between  the  external 
protective  structures  and  activity  ? 

3.  When  did  the  various  classes  of  mollusks  make  their 
appearance  in  the  history  of  the  earth  ?     (See  geology.)     What 
can  you  say  of  their  importance  in  the  formation  of  the  sedi- 
mentary rock  ? 

4.  In  what  ways  may  the  fresh- water  forms  have  arisen  from 
the  original  salt-water  mollusks  ? 

5.  What  members  of  the  group  of  mollusks  are  economically 
important  ?     Indicate  in  what  way  and  to  what  extent  ? 

6.  A  report  on  all  the  mollusks  to  be  found  in  your  com- 
munity; their  distribution,  habits,  etc. 

7.  Formation  of  pearls,  and  pearl  fisheries. 

8.  The  industries  connected  with  the  use  of  the  shells  of  the 
clam. 

9.  The  life  history  of  the  fresh- water  clam. 

10.  The  life  history  of  the  oyster. 


CHAPTER  XVII 

PHYLUM  XI.— ARTHROPODA 

304.  This  group  is  one  especially  favorable   for  the  pupils 
to  study  in  the  field,  in  the  haunts  of  the  animals  themselves. 
For  this  reason,  wherever  it  is  at  all  possible,  the  members  of 
the  class  should  be  required  to  collect  a  portion  of  the  material 
needed  in  the  laboratory  and  to  submit  a  report  on  such  items 
of  physiology  and  ecology  as  may  be  expedient  in  each  case. 
The  teacher  will  find  suggestions  in  the  supplementary  exercises. 

305.  The    Fresh -water    Crayfish    (Cambarus). — This  form 
should  be  studied  when  living  specimens  may  be  had.     They 
may  be  kept  for  considerable  time  in  a  tub  containing  an  inch 
of  water.     This  should  be  changed  every  day  or  two.     Feed 
on  small  pieces  of  meat  or  earthworms. 

I.  Physiology. 

1.  Locomotion:   walking;    how    effected?     swimming:   how 
effected?    Under  what  circumstances  does  the  animal  swim? 
Do  all  the  walking  legs  act  together  in  walking?     How  many 
are  at  rest  at  once  ?     In  what  order  do  they  act  ? 

2.  Movements  of  the  parts  of  the  body;  segments  and  ap- 
pendages.    Describe  the  manner  and  purpose  of  these  motions 
as  far  as  you  can  determine.     In  what  different  ways  do  the 
various  groups  of  appendages  seem  to  act?     Watch  them,  one 
pair  at  a  time. 

3 .  Feeding :  kind  of  food  used  and  manner  of  securing  it. 

4.  Respiration:  by  means  of  air  or  water?     How  can  you 
be  sure?     Does  the  animal   do  anything  to  renew  the  water, 
by  producing   currents?     Place  a  minute  amount  of  carmine 
or  indigo  solution  at  the  side  of  the  animal  at  the  union  of 
the  abdomen  and  thorax;  at  the  front  of  the  thorax.     What  is 
the  difference  ?     What  does  it  signify  ? 

5.  Evidences  of  sensitiveness:     Devise  experiments  of  your 
own  to  prove  whether   the    crayfish  is  stimulated  by  light; 
contacts;  the  presence  of  food  in  any  other  way  than  by  sight; 

265 


266  ZOOLOGY 

sound.  Are  all  parts  of  the  body  equally  sensitive  to  touch? 
To  chemical  stimuli?  Make  use  of  a  5  per  cent,  solution  of 
acetic  acid;  strong  salt  solution;  strong  beef  extract.  What 
inferences  may  be  drawn  from  your  experiments?  Place  a 
crayfish  on  his  back.  Describe  his  actions.  What  is  the  nature 
of  the  stimulus  that  arouses  this  reaction?  Evidences? 

II.  Symmetry. — (This  group  is  especially  favorable  for  this 
study.) 

Notice  what  is  implied  in  bilateral  or  tri-axial  symmetry. 

Antero-posterior  axis :  are  the  poles  alike  or  different  ? 

Make  a  memorandum  of  all  the  chief  differences. 

Dorso- ventral  axis  (as  above). 

Right-left  axis.     Record  the  points  of  agreement. 
Contrast  the  axes  in  length.     Can  you  think  of  any  causes 

for  the  differences  and  likenesses  discovered  above? 

Any  advantages  arising  therefrom  ? 

III.  General    Form. — Distinguish    two    regions; — Cephalo- 
thorax  and  abdomen. 

Cephalo-thorax ;  carapace. 
Head;  rostrum,  eyes,  mouth. 
Cervical  groove. 
Thorax. 

Abdomen;  how  many  segments  do  you  find?  What 
seems  to  determine  a  segment  ? 

Applying  these  criteria  can  you  find  any  indications  of 
segmentation  in  the  cephalo- thorax  ?  (Make  a  tem- 
porary estimate  of  the  number  of  segments  in  the 
animal.) 

Make  two  sketches  showing  a  dorsal  and  a  ventral  view 
of  the  crayfish,  preserving  proportions. 

Examine  one  of  the  abdominal  segments  (the  third  or 
fourth  from  the  front).  How  is  it  joined  to  those  next 
it  ?  Follow  the  line  of  union.  Note  tergum,  or  dorsal 
piece;  sternum,  or  ventral  piece;  pleura,  the  lateral  pro- 
jections from  the  tergum. 

Make  a  sketch  of  an  imaginary  cross  section  showing  the 
relation  of  these  parts  to  each  other,  together  with  the 
attachment  of  the  appendages. 


ARTHROPOD A  267 

IV.  Appendages.— Group  them  into  regions  and  notice 
the  general  differences  and  the  differences  in  the  uses 
to  which  they  are  put.  If  time  will  allow,  study  the 
appendages  in  detail  as  follows; 

1.  Begin  with  the  third  or  fourth  abdominal  appendage  (swimmerets)  making 
the  drawings  necessary  to  show  the  parts: 

Protopodite,  or  basal  portion. 

Exopodite,  or  external  branch. 

Endopodite  or  internal  (median)  branch. 

Compare  all  the  abdominal  segments  with  that  studied.  Do  different  indi- 
viduals agree  in  the  appearance  of  the  first  and  second  abdominal  segments? 
Compare  the  last  segment  (telson)  with  those  studied.  How  many  segments  in 
the  abdomen?  Of  what  parts  is  the  tail  fin  made  up? 

2.  Cephalo-thoracic    Appendages. — Remove    with    scissors    the    over-arching 
portion  of  the  carapace  and  expose  the  base  of  the  appendages.     Find  the  third 
maxilliped  (the  first  appendage  in  front  of 'the  large  claw).     Remove  by  inserting  a 
scalpel  and  bringing  away  all  that  belongs  to  it. 

Identify : 

Protopodite,  of  two  segments  (coxopodite,  next  the  body,  and  basipodite). 

Endopodite  and  exopodite.     How  many  pieces  in  each? 

Epipodite,  lying  in  the  gill-chamber.  Are  there  any  special  outgrowths  on 
it? 

Study  and  compare  with  this  the  large  claw,  and  the  other  walking  append- 
ages. Which  part  is  wanting  in  these,  exopodite  or  endopodite?  Reasons 
for  your  view?  How  do  these  five  appendages  differ  from  each  other. 

Examine  and  compare  the  appendages  in  front  of  the  third  maxilliped  in 
order: 

Second  maxilliped. 
First  maxilliped. 
Second  maxilla. 
First  maxilla. 


Mandible. 

Antenna. 

Antennule. 


Head  parts. 


What  are  the  evidences  that  the  antennae  and  antennules  are  homologous 
with  those  already  described  ? 

Revise  your  estimate  of  the  number  of  segments. 

Compare  the  appendages  again  by  groups,  and  notice  the  chief  points  of 
difference,  and  the  ends  served  by  these  differences.  Make  a  careful 
sketch  of  each  type  of  appendage,  labeling  all  parts.  (The  names  of  the 
segments  of  the  larger  appendages  may  be  found  in  fuller  texts,  if  desired.) 

By  studying  the  living  specimen,  determine  just  the  work  done  by  each 
of  the  types  of  appendages. 

Note  the  position  of  the  eyes.     Examine  with  a  low  power. 

In  the  basal  joint  of  each  antenna  is  the  opening  of  the  "green  gland." 

In  the  basal  joint  of  the  antennules  are  the  otocysts.     (Statocysts.) 


268  ZOOLOGY 

V.  Gills. — Examine  the  gill-chamber,  and  the  position  of 
the  gills  therein.  Which  appendages  bear  gills?  How  many 
tufts  to  each  appendage?  How  do  they  differ  as  to  the  place 
of  their  attachment  ?  How  many  in  all  ?  Make  a  table  showing 
these  factors. 

VI.  Internal  Organs. — Remove  with  much  care  the  carapace  from  the  thorax 
and  the  terga  from  the  abdominal  segments,  by  the  use  of  scissors  and  forceps. 
Sketch  the  organs  in  their  natural  position.     What  organs  are  visible? 

Examine  in  some  detail  the  following  sets  of  organs, 
(a)  The  circulatory  organs. 

Heart:  just  beneath  the  carapace,  in  a  membranous  chamber  (pericardial 

sinus). 
Apertures,  by  which  the  blood  enters  the  heart  from  the  sinus;  dorsal, 

ventral,  lateral.     How  many  do  you  find? 
Arteries;  anterior,  posterior. 

(The  teacher  should  have,  if  possible,  a  permanent  preparation  of  the  lobster 
in  which  the  arterial  circulation  has  been  injected  with  a  colored  mass.) 

VII.  Reproductive  Organs. — These  will    be  found  immediately  beneath  the 
pericardial  sac  as  whitish  (male),  or  yellowish  to  brown  (female)  lobed  structures. 
Depending  on  the  sex  there  will  be  found 

Ovaries  or  testes.     Form,  position,  and  number  of  lobes? 
Oviducts  or  vasa  deferentia.     Course,  length  and  outlets? 
Can  you  determine  the  sex  of  your  specimen?     Note  especially  the  external 
differences  between  males  and  females. 

VIII.  Digestive  Organs. 

Liver,  a  pair  of  yellow,  brown  or  reddish  masses  anterior  to  the  reproductive 
organs. 

Stomach;  sketch  in  position.  Dissect  later,  if  time  allows,  and  note  the 
anterior  and  posterior  chambers,  and  the  grinding  apparatus. 

How  is  the  mouth  situated  relatively  to  the  stomach  ? 

Follow  the  intestine  backward  from  the  stomach  to  the 
Anus:  position  of? 

Make  a  sketch  of  the  entire  tract  from  a  side  view,  showing  in  what  part  of  the 
carapace  each  portion  is. 

IX.  Muscular  System. — How  is  the  abdomen  flexed  and  how  extended?     How 
-do  the  muscle  fibres  run  ?     To  what  attached  ?     Are  they  plain  or  striate  ?     How  are 

the  appendages  worked?     Split  open  the  segments  of  the  chela. 

X.  Nervous  System. — (If  the  time  is  short  a  demonstration  may  be  made  by 
the  teacher,  preferably  with  a  lobster.) 

Remove  the  intestine,  and  cut  carefully  through  the  muscles  in  the  median 
line  until  the  white  ventral  nerve-chain  is  uncovered.  Follow  it  forward  to  the 
head,  cutting  away  the  covering  plates  in  the  thorax. 

How  many  swellings  (ganglia)  in  the  abdominal  region?  Relation  to  the 
segments?  Where  do  nerves  arise? 

Thoracic  ganglia:  number  and  relation  to  appendages? 

Sub-esophageal  ganglion;  circum-esophageal  connective. 

Supra-esophageal  ganglion  (brain). 


ARTHROPOD A  269 

Do  any  nerves  arise  from  the  brain?     Where  distributed?     Draw  from  above. 

Make  a  diagram  of  the  relation  of  the  digestive  tract  and  nervous  system  from 
the  side  view. 

XI.  Excretory  Organs. — The  green  glands  occur  at  the  base  of  the  head,  in  front 
of  the  mouth.  The  outlets  are  at  the  base  of  the  antennae. 

Make  a  diagrammatic  view  of  an  imaginary  cross  section  of  the  thorax  in  the 
region  of  the  heart,  and  one  of  the  abdomen,  showing  the  position  of  the  internal 
organs.  Also  a  diagram  of  a  sagittal  section  showing  relations  of  all  the  parts 
discovered. 

306.  Sow-bug  (Oniscus,  a  terrestrial  form;  or  Asellus,  a  fresh-water  Isopod). — 
General  Form. — Use  hand  lens  and  identify: 

Head:  size,  form,  number  of  segments. 
Eyes:  number  and  position. 
Antennules  and  antennae. 
Mouth-parts:  number  and  structure. 
Thorax:  number  of  segments.     What  variation  therein? 
Abdomen.     How  many  segments ?     Proofs? 
Appendages. 

Remove  carefully,  mount  in  water  on  a  slide,  and  examine  with  low  power, 

a  thoracic  appendage.     Sketch. 
Examine  similarly  the  other  thoracic  legs  and  the  mouth  parts,  and  make 

drawings  of  them  arranged  in  the  order  of  their  occurrence. 
Examine  similarly  the  abdominal  appendages.     What  is  their  number? 

Sketch? 
Compare  the  appendages  from  the  different  regions,  as  to  structure,  form 

and  probable  function.     Are  there  any  gills?     Where  situated? 
What!  s  the  number  of  segments  in  the  body,  if  there  is  a  pair  of  appendages 

to  each  segment? 

Comparisons. — Compare  the  sow-bug  with  the  crayfish  as  to  the  degree  of 
union  of  head  and  thorax;  the  number  of  segments  represented  in  each  of  the 
three  regions;  the  degree  of  differentiation  among  the  appendages;  the  mode  of 
respiration;  the  presence  of  both  exopodite  and  endopodite;  as  to  food,  and  habits. 
Physiology  and  Ecology. — A  study  and  report  of  the  animal's  habitat,  food 
habits,  methods  of  motion,  sensitiveness  to  light  and  to  other  classes  of  stimuli, 
should  be  made.  How  does  Oniscus  behave  when  touched?  Do  you  find  any 
trace  of  eggs  or  young?  What  facts  are  to  be  noted  concerning  them? 

307.  Cyclops. — These  minute  fresh-water  Crustacea  may  be  found  in  almost 
any  pool  where  aquatic  plants  are  found.     They  flourish  well  in  aquaria.     Select 
several  of  the  larger  specimens  with  egg  masses,  one  on  each  side  the  abdomen. 
Examine  in  a  watch  glass  with  a  little  water  to  which  a  drop  of  chloroform  has 
been  added.     Use  low  power  of  microscope. 

General  Form. — (Study  both  dorsal  and  ventral  surface.) 
Cephalo-thorax : 

Anterior  portion   covered   with   the  carapace.     How   many   segments 
represented?     How  can  we  know  that  this  is  not  merely  the  head,  or 
the  whole  cephalo- thorax? 
Posterior  portion  (four  free  thoracic  segments).     How  is  it  known  that 

these  are  not  abdominal  segments? 
Abdomen:  form;  number  and  character  of  the  segments. 


270  ZOOLOGY 

Appendages. — Antennae,  oral,  thoracic,  abdominal.  Number  and  general 
character  of  each.  Where  and  how  are  the  egg-cases  attached? 

Sense  Organs. 

Eye-spot  (appearing  as  one,  from  which  the  name  Cyclops). 
Do  you  find  any  organs  which  suggest  a  tactile  function? 

Report  on  all  available  points  of  physiology:  as  food  habits;  methods  of  locomo- 
tion; reaction  to  light  and  other  stimuli. 

308.  Comparisons. — Collect  all  the  minute  fresh-water  Crustacea  possible  and 
compare  them  with  Cyclops.  Learn  to  identify  them  by  their  manner  of  moving 
in  the  vessels  of  water.  Daphnia  is  especially  favorable  for  microscopic  study  on 
account  of  its  semi-transparency. 

309.  Spider  (any  common  species  large  enough  for  study). 
General  Form. — Study  the  relations  of  head,   thorax,  and 

abdomen.     Are  there  any  antennae  ?     Oral  appendages  ?     Num- 
ber and  character  of  the  thoracic  appendages  ?     Does  the  abdo- 
men show  any  signs  of  segmentation  ?     Has  it  any  appendages  ? 
Make  sketches  showing  a  ventral  and  a  lateral  view. 
Special  Organs. 

Examine  the  head  with  a  hand  lens  and  locate  the  eyes. 
Note  more  particularly  the  types  of  appendage  found, 
and  the  degree  of  differentiation.     Find  the  openings 
to  the  air-sacs  on  the  ventral  surface  of  the  abdomen. 
Locate  the  spinning  glands.     Number  ? 
Activities  and  Habits. — How  do  the  legs  act  in  walking? 
At  what  joints  are  they  flexed  at  various  parts  of  the  step? 
Do  all  the  legs  on  one  side  act  in  unison?     Observe  the  spin- 
ning action.     Does  the  spider  ever  produce  the  threads  except 
when  weaving  a  web?     Describe.     Determine  if  possible  the 
kind  of  web  formed  by  the  species  studied.     Or  find  as  many 
types  of  nest  or  web  as  possible  and  learn  to  recognize  the 
spiders  producing  them.     How  does  the  spider  travel  on  its 
web?     Where  do  spiders  place  their  webs?     Place  a  living  fly 
or  other  insect  in  a  newly  constructed  web  and  record  the 
actions  of  the  spider.     Can  you  devise  means  to  prove  whether 
the  spider  possesses  the  sense  of  smell  ? 

310.  The  Grasshopper. — Several  species  of  the  locusts  may 
be  found  in  almost  every  locality.     They  are  especially  abundant 
in  the  early  autumn.     For  laboratory  study  select  the  largest 
species  found  in  sufficient  abundance.     In  connection  with  the 


ARTHROPOD A  271 

securing  of  material  the  students  should  -make  observations  on 
the  following  points : 

1.  Habits. — Where  and  under  what  circumstances  found? 
At  what  time  of  the  year  does  this  species  occur  in  greatest 
abundance  ?     Under  what  circumstances  are  they  most  active  ? 

2.  Methods    of    Locomotion. — How    many    methods    seem 
available?     Degree  of  efficiency  of  each?     Under  what  circum- 
stances is  each  used?     What  distance  can  be  attained  at  one 
effort?     Continue  the  study  later  in  more  limited  quarters,  as 
in  the  room  and  under  a  bell-glass.     Compare  the  work  of  the 
various  legs.     Are  the  wings  used  at  all  in  jumping  ? 

3.  Protective  Features. — Coloring;  to  what  extent  do  you 
find  this  of  protective  value  ?     Reasons.     Does  the  animal  show 
a  distinct  instinct  for  hiding  ?     Compare  all  available  species  in 
these  regards. 

4.  Do  they  produce  definite  sounds?     Under  what  circum- 
stances ?     Do  you  find  any  hint  as  to  the  method  of  their  pro- 
duction ? 

5.  Do  you  detect  any  movements  which  suggest  respiration? 
Rate  ?     (Find  spiracles  in  the  thorax  and  abdomen.) 

6.  Supply  hungry  animals  with  fresh  leaves  and  study  the 
feeding  process.     Dip  the  leaves  in  various  solutions  and  notice 
whether  it  makes  any  difference  to  the  grasshopper. 

If  alcoholic  material  is  used  for  the  following  morphological 
studies  it  should  not  be  allowed  to  become  dry.  If  dipped  in 
a  mixture  of  glycerine  and  50  per  cent,  alcohol,  specimens  will 
not  dry  so  rapidly. 

The  sexes  differ,  particularly  in  the  abdominal  region.     Pro- 
cure specimens  thus  differing  by  examining  a  number  of  individuals, 
and  keep  both  kinds  for  comparison.     Sketch  dorsal,  lateral  and 
ventral  views  of  each  (especially  in  the  regions  of  difference). 
External  Features. — Study  the  following  points : 
i.  The  regions  of  the  body. 
Head;  thorax;  abdomen. 

What  are  the  signs  of  segmentation  in  these  three  regions  ? 
Where  is  it  most  clearly  indicated?  Where  are  the 
segments  most  similar? 


272  ZOOLOGY 

2.  Abdomen. 

Number  of  segments  (differs  in  male  and  female). 
Dorsal  and  ventral  plates.     Are  they  equally  developed  in 

all  segments. 
Appendages :  which  segments  possess  them  ? 

Ovipositors  (paired  outgrowths  found  only  in  the  female). 

Anal  cerci  (examine  the  male).     Are  they  found  in  the 

female?     To  what  segments  do  these  appendages 

belong  ? 

Spiracles  (small  openings  at  the  side  of  the  segments); 

number  and  distribution  ? 

Tympanic  membrane,  at  the  sides  of  the  first  abdominal 
segment. 

3.  Thorax:  studying  from  the  front,  backward,  find: 
Prothorax;    mesothorax;    metathorax.     Note    the  form, 

size,  and  structure  of  each  part. 
Appendages  of  each  segment. 

Legs:  number;  relative  size;  parts  (beginning  at  the 
body),  coxa,  trochanter,  femur,  tibia,  tarsus.  Com- 
pare the  legs. 

Wings  (can  these  be  regarded  as  homologous  with  the 
jointed    appendages?):    number;    position,   at  rest 
and  in  motion;   characteristics;  position  of  veins. 
Compare  the  two  pairs  in  all  essential  particulars. 
Are  there  any  spiracles  in  the  thorax  ?     Position  ? 

4.  Head  (is  there  any  "neck"  ?).     The  head  is  covered  with 

chitinous  plates ;  identify : 

Epicranium,  the  dorsal  plate. 

Clypeus,  the  anterior  plate. 

Genae,  the  lateral  plates. 

Labrum  or  upper  lip,  anterior  to  the  clypeus. 
Examine  the  compound  eyes,  their  form  and  relation  to  the 

plates.     Slice  off  a  portion  of  the  surface  and  study  the 

surface  with  a  low-power  objective. 

Ocelli  or  simple  eyes.     How  many  and  in  what  position? 
Mouth  aperture;  position. 
Appendages  of  the  head : 

Antennae,  near  the  eyes;  number. 


ARTHROPODA  273 

Mouth-parts.  These  are  complicated  and  demand  careful  study,  if  satis- 
factorily made  out.  Remove  the  labrum  and  proceed  from  before, 
backward. 

Mandibles;  a  pair  of  horny  tooth-bearing  jaws.     Draw  in  position. 
Maxilla;  a  pair  of  compound  jointed  organs  made  up  of  three  portions, 

the  lacinia  (nearest  the  median  line),  the  galea,  and  the  maxillary 

palpus  (external). 
Labium  or  lower  lip;  this  also  bears  a  palpus.     The  labium  may  be 

studied  and  removed  before  the  study  of  the  maxillae. 
Tongue. 

How  many  segments  seem  to  be  represented  in  the  head?     Evidences. 
Internal  structure. 

Select  large  female  specimens  preferably.     Clip  the  wings  close  to  the 

body,  and  pin  the  specimen  to  a  board,  dorsal  surface  upward. 
With  a  pair  of  fine,  sharp  pointed  scissors  make  a  longitudinal  incision 

into  the  integument  of  the  abdomen  near  each  side.     Gradually  and 

carefully  remove  the  skin  between  the  cuts  from  behind  forward. 

Look  for  the  heart, — a  long,  thin-walled,  mid-dorsal  vessel,  which  if 

not  removed  with  the  skin  may  be  seen  just  beneath  it.     Unroof  both 

the  abdomen  and  thorax.     Note  the  exposed  muscles  of  the  thorax, 

also  the  whitish  fat  bodies  next  the  body  wall. 

1.  Trachea. — If  the  specimen  is  freshly  killed  the  tracheae  will  be  filled  with 
air  and  will  show  as  white,  glistening  tubes.     Seek  their  connection  with 
the  spiracles,  and  note  their  ramification  and  unions  in  the  body.     Isolate 
some  of  the  smaller  branches  and  study  under  the  microscope.     Prove 
that  they  are  tubes.     How  kept  open? 

2.  Reproductive  Organs. — (These  are  much  more  difficult  in  the  male.) 
Ovaries:  In  how  many  masses?     Notice  the  subdivisions  of  the  ovaries. 

These  contain  the  eggs  and  communicate  by  means  of  an  oviduct 
with  the  outside.  In  what  segment?  Examine  an  ovum  with  the 
microscope.  Mash,  and  notice  the  yolk. 

3.  How  do  the  muscles  of  the  thoracic  region  differ  from  those  in  the 
abdominal?     Are  the  fibres  plain  or  striate? 

4.  Digestive  Tube. 

Dissect  forward  into  the  head,  and  press  the  other  organs  aside  so  that 
the  course  of  the  tract  may  be  revealed.     It  consists  of  the  following 
parts,  which  should  be  identified: 
Mouth. 

Esophagus;   size  and  course. 

Crop  (an  enlargement  of  the  esophagus);  shape,  position. 
Stomach;  character  and  extent.     (At  the  anterior  end  is  a  ring  of 
tubular  appendages  which  are  glandular  in  function, — the  gastric 
caeca;  at  the  posterior  end  it  is  limited  by  a  circle  of  fine  tubes — 
Malpighian  tubules — which  are  excretory.) 
Intestine;  length,  course  and  size. 
Anal  opening;  position. 

Make  drawing  of  digestive  tract  from  side  view,  showing  in  outline  the 
body  regions  and  the  relation  of  the  portions  of  the  tract  to  these. 

5.  Nervous  System. — (Remove  the  alimentary  tube  and  examine  the  floor 

of  the  abdominal  cavity.) 
18 


274  ZOOLOGY 

Ventral  nerve  cord.     Is  it  single  or  double? 
Ganglia;  number,  and  relation  to  the  segments. 
Nerves;  origin  and  distribution. 
Trace  forward  into  the  thorax  and  head. 
Ganglia;    number    and    position.     How    connected?     Is    there    any 

portion  dorsal  to  the  digestive  tract  (brain)  ? 
Nerves. 
Compare  the  nervous  system  of  the  grasshopper  part  by  part  with  that 

of  the  crayfish. 

Make  diagrammatic  representations  of  imaginary  cross  sections  through 
thorax  and  abdomen  showing  the  relation  of  the  different  structures; 
likewise  of  a  sagittal  section. 

The  cricket  or  cockroach  may  be  substituted  for  or  compared  with  the  grass- 
hopper. 


311.  Supplementary   Laboratory   and   Field   Work. — It   is 

perhaps  inexpedient  for  students  in  an  elementary  course  to 
make  dissections  of  other  representatives  of  the  Arthropoda, 
but  the  common  air-breathing  forms  are  so  numerous,  so  varied, 
and  have  such  interesting  habits  and  histories,  that  they  may 
profitably  be  used  as  a  basis  for  individual  field  and  laboratory 
work  and  to  serve  in  the  comparison  of  homologous  organs  in 
related  groups.  The  following  outlines  are  suggestive  rather 
than  exhaustive. 

I.  Make  a  table  in  which  can  be  displayed  the  points  of  con- 
trast between  the  crayfish,  the  grasshopper,  the  "June  bug" 
(or  other  beetle),   the  squash-bug  or  the  cicada   ("locust"), 
the  butterfly,  the  wasp,  the  fly,  the  spider,  and  the  centipede,  in 
the  following  particulars: 

1.  The  regions  of  the  body;  head,  thorax,  and  abdomen; 
their  degree  of  development  and  separateness. 

2.  The  number  of  segments  in  the  body,  and  the  clearness 
with  which  they  are  manifest. 

3.  The  degree  of  diversity  shown  by  the  segments  in  the 
various  parts  of  the  body. 

4.  The   points   of   structure   which   the   various   segments 
possess  in  common. 

II.  Make  a  similar  table,   for  the   same  animals,   of   the 
appendages : 

i.  Head  appendages:  antennae;  mouth  parts,  number  and 
kinds. 


ARTHROPOD A  275 

2.  Thoracic  appendages: 

Legs:  number,  position,  kinds,  joints,  special  adaptations 

to  special  work. 
Wings:  number,  size,  position,  structure,  principal  veins. 

Compare  the  first  and  second  pairs  as  to  size,  structure 

and  function. 

3.  Abdominal  appendages:  number,  structure,  function. 

III.  Make  a  table  comparing  these  and  other  available  forms 
as  to  their  eyes,  simple  and  compound. 

IV.  Find,  if  possible,   another  form  embodying  the  same 
general  features  found  in  each  of  the  above-mentioned  animals. 

V.  Compare  these  (or  other  forms  which  may  be  selected) 
from  the  point  of  view  of  their  habits.     Introduce  all  discovered 
correlation  between  structure  and  function. 

1.  Haunts  and  place  of  living.     If  peculiarly  local,  can  you 
find  any  reasons  ? 

2.  Locomotion:  methods,  and  the  efficiency  of. 

3 .  Feeding :  material  used,  and  the  method  of  obtaining  it. 

4.  Respiration:  organs  and  their  location;  any  special  points 
as  to  their  use. 

5.  Special    senses:    physiological    evidences;    morphological 
evidences. 

6.  The  laying  of  eggs  and  provision  for  the  young. 

(The  library  may  be  used  profitably  to  supplement  field 
work  in  this  section.) 

VI.  Study  by  observation  the  homes,  temporary  or  perma- 
nent, their  mode  of  construction  and  uses,  in  the  following: 
Spiders   (as   many   species   as  possible),    the  paper- wasp,    the 
mud-wasp,  the  honey-bee,  the  bumble-bee,  ants,  flies,  etc. 

VII.  Development  or  life  history.     Studies  may  be  made  in 
natural   conditions  in   many   cases   by   periodic   observations. 
When  this  is  not  possible,  animals  may  often  be  reared  in  con- 
finement by  supplying  the  appropriate  conditions.     This  is  a 
very  attractive  line  of  investigation  and  one  in  which  real 
contributions  to  knowledge  may  be  made.     The  following  are 
some  of  the  matters  to  be  kept  in  mind. 

i .  Is  there  a  metamorphosis  or  is  development  direct  ?     (See 
text,  §330.) 


276  ZOOLOGY 

2.  Eggs:  where  deposited ?     In  what  numbers ?     Relation  to 
future  food  supply  ? 

3.  Larval    condition    ("grub,"    "maggot,"    "caterpillar"). 
Form,  segmentation,  general  external  characters,  special 
organs;  habits,  food,  coloration,  enemies. 

4.  Pupa    (a    resting    and    transforming    stage) ;    how    pro- 
tected?    What  is  the  origin  and  character  of  the  pro- 
tecting structure?     What  changes  are  undergone  at  this 
stage  ? 

5.  Adult.     How  do  the  larva  and  pupa  compare  with  it  in 
segments,  appendages,  etc. 

The  following  forms  may  be  studied  and  compared  as  to  life 
history : 

Squash-bug;  all  stages  are  to  be  found  on  squash,  gourd, 
cucumber  and  similar  vines. 

Potato  beetle;  equally  abundant  on  the  Irish  potato  plant 
in  some  years. 

Bees  and  wasps;  to  be  found  in  their  nests. 

"Blue-bottle"  fly.  This  form  may  be  studied  in  confine- 
ment. (Expose  raw  meat  for  the  eggs  to  be  laid. 
Place  on  a  chip  in  a  dish  of  moist  earth  or  sand.  Invert 
a  tumbler  or  bell- jar  over  it  and  watch  the  growth  and 
changes,  as  decomposition  proceeds.) 

Mosquito.  The  Iarva3  may  be  found  in  stagnant  pools, 
and  watched  in  confinement. 

Cabbage  butterfly.  This  form  may  be  studied  in  the 
garden,  or  in  the  laboratory  by  placing  the  cabbage 
leaves  with  the  larvae  under  a  bell- jar  and  keeping  the 
conditions  favorable. 

Some  large  caterpillar  should  be  studied  with  some  de- 
gree of  care  in  order  to  ascertain  the  general  arrange- 
ment of  organs. 

Spider.  If  a  mass  of  spiders'  eggs  can  be  found,  the  stu- 
dent by  watching  may  be  able  to  determine  whether 
the  development  is  direct  or  indirect. 

Silk-worm.     The  various  stages  may  be  studied  in  con- 
finement. 
VIII.  Group  the  Arthropoda  known  to  you,  in  three  classes: 


ARTHROPOD A  277 

(i)  those  hurtful  to  man's  interests,  (2)  those  beneficial  thereto, 
and  (3)  the  harmless.  State  the  grounds  of  your  classifica- 
tion of  each  form.  In  what  stage  of  its  metamorphosis  is 
each  species  hurtful  or  helpful.  Extend  your  own  knowledge 
by  inquiry,  by  observation,  and  by  reading. 

DESCRIPTIVE  TEXT 

312.  The  group  of  Arthropoda  (jointed-legs)  embraces  more 
than  one-half  the  species  in  the  animal  kingdom,  and  is  cor- 
respondingly rich  in  individuals.     This  is  the  same  as  saying 
that  they  are  remarkably  variable  and  adaptable  to  various 
conditions  of  life.     The  segmented,  bilaterally  symmetrical  body 
and   the   arrangement   of   the   nervous   system   are   the   most 
important  points  of  similarity  with  the  Annulata.     The  general 
resemblance  is  more  striking  in  some  of  the  lower  forms  (Peri- 
patus),  and  in  the  larval  stages  of  those  which  undergo  a  meta- 
morphosis.    The  subdivisions  of  the  phylum  (if  it  can  be  con- 
sidered a  single  phylum)  are  quite  diverse  and  their  relation- 
ships   uncertain.     There    are    many    parasitic    and    otherwise 
degenerate  forms   which   make   the   problem   of   classification 
more  difficult. 

313.  General  Characters. 

1.  Elongated,  bilaterally  symmetrical  body. 

2.  Segmented;  somites  heteronomous,  and  typically  grouped 
into  three  regions:  (i)  head,  (2)  thorax,  (3)  abdomen. 

3.  An  outer  skeleton,  of  a  secreted  chitinous  substance. 

4.  Each  somite  has  typically  a  pair  of  jointed  appendages 
(whence  the  name  arthropod). 

5.  Central  nervous  system  similar  to  that  of  Annulata:  (i) 
brain,   (2)  a  nerve  ring  around  the  esophagus  connecting  the 
brain  with  (3)  a  ventral,  ladder-like  chain  of  ganglia. 

~6.  Heart,  dorsal  to  the  digestive  trace. 

7.  Ccelom  Tepresented  largely  by  secondary  blood  spaces 
connecting  with  the  circulatory  system. 

314.  General  Survey. — The  symmetry  of  the  Arthropods 
is  very  pronounced,  except  in  the  case  of  fixed,  parasitic,  or 
otherwise    degenerate    forms    (as    barnacles,    Sdcculina,    etc.). 


278  ZOOLOGY 

The  group  presents  great  diversities  expressive  of  a  high  de- 
gree of  adaptation  to  almost  every  conceivable  mode  of  life. 
They  may  be  parasitic — internal  or  external,  symbiotic,  social, 
or  independent;  they  may  be  aquatic,  terrestrial,  burrowing  or 
aerial;  they  use  all  sorts  of  food;  they  bore,  crawl,  swim,  jump, 
fly,  or  may  be  fixed.  In  geographical  distribution  they  are 
practically  cosmopolitan.  The  group  is  one  of  the  most  suc- 
cessful in  the  animal  series.  None  of  the  living  species,  however, 
attains  a  very  great  size.  The  king-crab  and  the  lobster 
are  among  the  largest.  Many  are  microscopic. 

315.  The   Segments. — There  is  a  great  deal  of  diversity 
among  the  segments  of  the  body  as  to  size,  shape,  the  form 
and  use  of  their  appendages,   as  well  as    in    their  contained 
structures.     In  the  more  primitive  forms  (Peripatus  and  the 
centipedes)  and  in  the  larval  condition,  the  somites  are  well 
marked  externally,  but  in  the  majority  of  forms  there  is  more 
or  less  fusion  of  contiguous  somites  in  certain  body  regions. 
A  variable  number  of  segments  at  the  anterior  end,   which 
bear  the  mouth  parts  and  sense  organs,  form  the  head.     Be- 
hind these  a  group  of  three  (insects),  or  more  (crayfish),  may 
fuse   to   form    the    thorax.     These    two    regions,    head    and 
thorax,    are    often    fused    into    one    piece — the    cephalotkoraoo. 
The  abdominal  segments  are  usually  unfused. 

316.  The   Appendages   also   differ   much   in   form   in   the 
various  representatives  and  on  different  segments  of  the  same 
individual.     This  diversity  of  structure  is  closely  connected 
with  the  variety  of  work  to  be  done,  and  is  an  excellent  illus- 
tration of  the  differentiation  which  accompanies  "division  of 
labor.*'     They  are  unquestionably  serially  homologous  organs 
as  is  shown  by  their  similarity  of  origin  and  by  the  funda- 
mental likeness  of  structure, — clearly  to  be  seen  in  the  primi- 
tive forms.     They  may  be  said  to  consist  typically  of  a  basal 
portion  with  one  or  more  segments,   supporting  two  jointed 
branches, — a  median  and  an  external.     Appendages  may  be 
entirely   wanting    (as   in   the  abdominal   segments  of  insects) ; 
and  yet  these  may  appear  in  a  rudimentary  form  in  the  early 
stages  of  the  embryo,   only   to  disappear  later.     Where  the 
metamerism  is  obscured  by  fusion,  the  number  of  appendages 


ARTHROPOD  A  279 

may  be  the  only  indication  we  have  of  the  number  of  segments; 
but  as  we  have  seen,  the  appendages  themselves  are  some- 
times aborted  in  regions  where  they  are  no  longer  needed. 
So  it  is  not  always  possible  to  determine  how  ma^Sy  segments 
are  really  represented  in  an  animal. 

General  groups  of  appendages  are  as  follows:  (i)  preoral, 
mostly  sensory, — as  antennae;  (2)  oral,  biting  and  sucking 
structures, — mandible  and  maxillae;  (3)  thoracic,  chiefly  walk- 
ing appendages;  (4)  abdominal,  variously  modified  (as  swim- 
merets,  gills,  etc.),  or  wanting.  The  wings  are  not  to  be  re- 
garded as  homologous  with  the  jointed  appendages.  They 
originate  as  expansions  of  the  integument  of  the  body,  sup- 
ported by  numerous  tubular  ribs  or  "veins"  containing  branches 
of  the  blood  vessels,  tracheae,  and  nerves.  Wings,  when 
present,  comprise  one  (flies)  or  two  pairs  (bees).  Often  the 
anterior  pair  is  hardened  and  serves  merely  as  a  protection 
for  the  second  pair.  Either  pair,  more  often  the  second,  may 
be  aborted. 

317  Ccelom. — The  development  of  the  arthropods  shows  that  the  spaces  in 
the  body  are  not  truly  coelomic  as  a  rule,  but  are,  so  to  speak,  much  enlarged 
blood  spaces  containing  the  corpuscle-bearing  fluid.  The  pericardial  sinus  is 
one  of  these.  Such  a  body  cavity  is  known  as  a  hamoccele. 

318.  Integumentary  Structures. — The  arthropod  skin  has 
an  epidermal  layer  of  cells  which  secretes  the  chitinous  cuticle 
constituting  the  external  skeleton.  The  chitin  may  be  mixed 
with  salts  of  lime.  Beneath  the  epidermis  is  a  layer  of  con- 
nective tissue, — the  dermis,  containing  nerves  and  blood 
vessels.  Still  within  these  are  the  longitudinal  muscles  of  the 
body  wall.  This  chitinous  covering  serves  for  protection 
and  support  of  the  soft  parts,  and  for  the  attachment  of  the 
muscles  of  locomotion.  When  the  secreted  shell  becomes  thick 
and  hard,  further  growth  is  necessarily  more  difficult.  This  diffi- 
culty is  usually  overcome  by  moulting,  in  which  process  the  old 
cuticle  is  separated  from  the  epidermis,  rupturing  along  some 
line  of  weakness,  and  allowing  the  escape  of  the  animal.  This 
moulting  extends  not  only  to  the  minutest  of  the  external 
organs,  but  to  the  stomodaeum  and  prqctodaeum  as  well.  A 
new  cuticle  begins  to  be  secreted  at  once,  but  this  "soft-shelled" 


280  ZOOLOGY 

condition  is  one  of  great  danger  and  helplessness  to  the  animal. 
The  process  besides  is  exhausting,  and  to  these  facts  we  may 
attribute,  in  part  at  least,  the  small  size  of  most  arthropods. 
The  cuticul$  is  laid  down  very  thinly  at  the  joints.  Thus  is 
secured  the  flexibility  necessary  in  locomotion. 

319.  Muscles    and    Locomotion. — The    muscles    are    well 
developed,  and  many  of  the  arthropods  are  very  powerful  in 
proportion  to  their  size.     The  circular  muscles  characteristic 
of  the  Annulata  are  lacking  in  the  arthropods.     The  chief  body 
muscles  are  the  longitudinal  which  cause  the  flexion  and  ex- 
tension of  the  segments.     There  are  in  addition  the  muscles  by 
which  the  appendages  are  moved.     These  fibres  are  of  the  cross- 
striate  type.     Less  massive  groups  of  fibres  are  found  in  the 
walls  of  portions  of  the  digestive  tract. 

The  paired,  segmented  appendages  are  primarily  organs  of 
locomotion.  They  are  variously  used  for  crawling,  walking, 
jumping,  and  swimming.  Many  have  wings,  and  the  quality 
of  the  nervous  and  muscular  control  of  these  can  be  realized  from 
the  fact  that  the  bumble  bee  can  make  some  240  wing  strokes 
per  second  and  the  fly  330  per  second.  Some  forms,  as  the 
crayfish,  have  the  power  of  using  segmented  portions  of  the 
body  in  a  powerful  backward  swimming  stroke.  Some  types 
have  the  power  of  skating  round  on  the  surface  of  water. 
Locomotion  in  the  phylum  is  remarkably  varied  and  remarkably 
well  done. 

320.  Digestion  and  the  Digestive  Organs. — The  alimentary 
tube  is  typically  rather  complex.     It  commences  with  a  mouth 
which  is  usually  supplied  with  three  or  more  pairs  of  external 
appendages  assisting  in  the  capture,  transfer,  and  preparation 
of  food.     This  is  followed  by  an  esophagus    either   with  or 
without   a   crop ;   a   stomach   frequently  consisting   of  several 
regions:  viz.   (a)  a  proventriculus  or  gizzard  in  which  food  is 
further  broken  to   pieces  physically,  and  (fr)  a  ventriculus  or 
stomach  proper  in  which  chemical  digestion  occurs ;  an  intestine 
which  is  not  always  clearly  marked  off  from  the  stomach;  and  a 
posterior  opening, — the  anus.     The  development  of  the  gut  shows 
both  stomodseum  and  protodaeum  (see  §93).     The  former  is 
often  very  extensive, — embracing  even  the  pro  ventriculus,  in 


ARTHROPOD  A  281 

.which  chitinous  grinding  plates  may  occur  (crayfish,  cockroach). 
The  "salivary"  glands  when  present  open  into  the  esophagus 
or  mouth  cavity.  Into  the  mesenteron  important  digestive 
glands  may  open,  as  the  pyloric  caeca  (many  insects),  or  liver 
(crayfish  and  spiders).  The  Malpighian  tubules  (see  dissec- 
tion of  the  grasshopper)  associated  with  the  hind  gut  are 
believed  to  be  excretory  rather  than  digestive  in  function. 
The  digestive  system  as  a  whole  is  strikingly  correlated  with 
the  character  of  food  used,  which  is  exceedingly  diversified 
in  this  phylum.  This  results  in  the  fact  that  the  details  of 
structure  are  scarcely  the  same  in  any  two  species.  Indeed  the 
digestive  process  and  structures  are  very  different  in  the  larval 
and  adult  stages  of  the  same  individual, — as  in  the  caterpillar 
that  feeds  on  green  leaves  and  the  adult  that  lives  on  nectar  or 
fruit  juices.  This  can  be  appreciated  only  by  extended  ob- 
servation and  comparison.  The  student  is  urged  to  com- 
pare such  figures  of  these  organs  as  he  may  be  able  to  find  in 
the  reference  texts  at  his  command. 

321.  Respiration. — In  some  instances  the  Arthropoda  ob- 
tain their  oxygen  directly  from  the  air,  in  others  from  the 
water.  In  the  latter  the  exchange  is  effected  through  the  body 
wall,  or  by  gills.  These  are  essentially  thin  outgrowths  of 
the  body  wall,  with  the  cuticula  much  reduced  or  absent, 
into  which  the  blood  passes  (e.g.,  the  majority  of  Crustacea). 
In  the  former  it  takes  place  wholly  by  means  of  tubular  air- 
passages  or  trachea  (insects),  or  these  may  be  supplemented  by 
thinned  folds  of  the  body  wall — book-lungs  (spiders).  By 
these  devices  the  oxygen  of  the  air  or  water  and  the  blood 
are  brought  into  intimate  relations.  In  the  water-breathing 
forms  the  gills  are  either  the  modified  appendages  (Limulus, 
Asellus},  or  specialized  outgrowths  from  them  or  from  the 
general  body  wall  (crayfish;  Fig.  126,  g).  The  gills  vary 
widely  in  number  and  position,  but  are  found  especially  in 
connection  with  the  thoracic  and  abdominal  appendages. 
The  air-breathing  forms  possess  a  system  of  interbranching 
tubes  which  may  open  to  the  exterior  by  a  pair  of  stigmata  or 
pores  in  each  somite.  These  tubes  unite,  branch  and  penetrate 
to  every  portion  of  the  body.  The  air  is  carried  to  the  blood 


282 


ZOOLOGY 


rather  than  the  blood  to  the  air.  The  tubes  are  lined  by  a 
thin  layer  of  cuticle,  and  are  kept  open  by  a  spiral  thread  of 
the  same  material  reinforcing  the  wall.  The  book-lungs  when 
present  lie  within  a  sac  which  opens  to  the  exterior  by  a  stigma 
or  pore,  and  consist  of  a  series  of  pleatings,  within  which  the 
blood  circulates  and  between  which  the  air  circulates. 

The  larvae,  especially  of  air  breathers,  are  often  developed 
in  conditions  very  different  from  those  chosen  by  the  adults. 

FIG.  126. 


PlG.  126.  Diagrammatic  cross  section  of  Crayfish  in  the  thoracic  region,  to  show  relation 
of  circulation  and  respiration,  a,  appendage;  c,  carapace;  e.f.,  flap  of  carapace  overhanging  the 
gills;  d,  digestive  tube;  g,  gill;  h,  heart;  /,  liver;  m,  body  muscles;  m',  muscles  of  the  appendages; 
n.c.,  nerve  cord;  p.s.,  pericardial  sinus;  r,  reproductive  glands;  st,  sternal  artery;  r.a.,  ventral  artery; 
v.s.,  ventral  blood  sinus  in  which  the  nerve  cord  lies.  Modified,  from  Lang. 

Questions  on  the  figure. — What  is  the  relation  of  the  gills  to  the  body  wall? 
Follow  the  course  of  the  circulation  by  the  arrows.  It  leaves  the  heart  by  definite 
arteries  and  comes  back  by  less  definite  blood  sinuses.  What  is  the  function  of 
the  valves?  What  gain  is  there  in  the  position  of  the  ventral  nerve  cord  in  the 
blood  sinus? 


This  fact  may  make  necessary  very  important  changes  in  the 
respiratory  organs  in  the  metamorphosis.  Some  forms  are 
even  water  breathers  in  the  larval  stage  and  air  breathers  in 
the  adult  (dragon-flies). 


ARTHROPOD  A  283 

322.  Circulation. — The    heart    or    pulsating    organ    when 
present  is  dorsal  and  may  be  much  elongated,  with  an  enlarge- 
ment in  each  somite.     It  lies  in  a  membrane-bounded  cavity 
called  the  pericardial  sinus  (Fig.  126,  ps.),  which  is  a  part  of 
the  haemoccele   or  secondary   body  cavity  (§317).     The  blood 
comes  to  the  pericardial  cavity  and  enters  the  heart  by  means 
of  slit-like  openings,  with  valves.     Definite  arterial  vessels  leave 
the  heart  and  pass  to  capillary  regions  and  thereupon  open 
into  irregular  spaces  in  the  tissues  without  definite  walls  (lacuna) . 
The  hsemoccele  is  in  reality  an  enlarged  lacuna.     In  insects 
there  is  an  anterior  artery  only;  in  spiders  and  Crustacea,  pos- 
terior and  lateral  arteries  also  occur.     The  return  of  the  blood 
takes  place  through  the  irregular  haemoccele  spaces  (lacunae). 
These  become  more  definite  in  form  as  they  near  the  pericardial 
chamber,  or  as  they  approach  the  gills  in  aquatic  forms.     One 
of  the  more  important  blood  spaces  is  the  ventral,  in  which  the 
nerve  cord  lies  (Fig.  126,  v.s.).     The  blood  corpuscles  are  color- 
less and  amoeboid.     The  plasma  may  be  variously  colored  by 
pigments  which  seern  to  assist  in  the  work  of  respiration,  as 
haemoglobin  does  in  vertebrates. 

323.  Excretion. — The    importance    of    excretion    increases 
with  the  activity  of  animals.     Except  in  Peripatus  it  is  not 
conclusive  that  any  of  the  adult  excretory  organs  in  this  phylum 
are  homologous  with  the  segmental  organs  of  Annulata.     In 
insects  and  spiders  there  are  excretory  tubules  communicating 
with  the  hind  gut.     In  the  crayfish  and  related  forms  a  pair 
of  excretory  glands — "green  glands" — open  at  the  base  of  the 
antennae.     It  is  of  importance  to  remember  that  the  exoskeleton 
of  the  Arthropoda  is  an  excretion,  which  is  incidentally  protective 
and  supportive. 

324.  The  Nervous  System- consists  essentially  of  the  same 
parts  as  have  been  described  for  the  annelids.     It  is,  however, 
on  the  whole,  more  fully  developed.     This  development  accords 
with  the  differentiation  which  we  have  seen  in  the  somites  and 
body   regions.     The    brain    and    sub-esophageal    ganglia,    for 
example,  have  become  more  pronounced  with  the  differentia- 
tion of  the  head;  accompanying  the  fusion  of  the  body  seg- 


284  ZOOLOGY 

ments  there  is  a  massing  and  fusion  of  the  corresponding  ganglia; 
and  in  general,  everything  considered,  those  ganglia  are  best  de- 
veloped which  lie  in  the  best-developed  somites.  The  concen- 
tration of  the  ganglia  of  the  ventral  cord  may  continue  until 
they  form  practically  one  mass.  Nerves  arise  from  the  brain, 
from  the  connective  about  the  gullet,  and  from  the  ventral 
ganglia,  and  pass  to  sensory  organs  and  to  the  muscles  of  the 
body  and  the  appendages. 

325.  Organs  of  Special  Sense. — As  the  thickened  cuticular 
covering  of  the  arthropods  develops,  it  is  apparent  that  much 
of  the  sensitiveness  of  the  surface  to  external  conditions  must 
be  lost  unless  special  structures  are  produced  to  compensate 
for  this  by  the  reestablishment  of  connection  between  the 
internal  organs  and  the  outside  world.  Such  structures  we 
find  in  the  chitinous  hairs  of  various  shapes  which  project  be- 
yond the  surface  and  in  pits  or  canals  which  pierce  the  skeleton. 
These  all  have  nervous  connections  and  have  been  variously 
interpreted  as  tactile,  taste,  auditory,  and  olfactory  organs. 
They  are  especially  abundant  in  the  more  movable  portions, 
particularly  those  about  the  mouth,  though  they  may  be  found 
over  the  whole  surface  of  the  body.  Figures  illustrating  the 
great  variety  of  forms  of  such  hairs  should  be  sought  in  the 
reference  texts. 

It  has  been  shown  that  the  crayfish  and  many  insects  are 
sensitive  to  chemical  stimuli.  These  sensations  have  to  do  with 
selection  of  food  and  with  testing  the  character  of  the  water 
or  air.  They  are  analogous  to  our  sensations  of  taste  and  smell. 
The  organs  may  be  scattered  over  the  body  or  more  commonly 
localized  on  the  appendages  about  or  in  front  of  the  mouth. 

At  least  three  classes  of  organs  have  been  described  as  audi- 
tory among  the  arthropods :  (a)*  vibratile  hairs,  as  in  the  case 
of  the  male  mosquito  (Fig.  42);  (b)  otocysts,  as  in  many  aquatic 
forms;  (c)  a  tympanum  or  membrane  in  connection  with  which 
are  special  nervous  cells  for  the  reception  of  the  vibrations  (as 
in  the  grasshopper  and  other  insects) .  The  otocysts  of  the  Crus- 
tacea may  be  open  or  entirely  closed.  In  the  former  case  the 
animal  itself  may  place  the  otoliths  in  the  otocyst  in  the  form 


ARTHROPODA 


FIG.  127. 


of  grains  of  sand.  Recent  investigations,  however,  show  that 
the  function  of  this  organ  is  not  hearing,  so  much  as  that  of  in- 
forming the  animal  of  its  relation  to  the  pull  exerted  by  gravity, 
thus  enabling  it  to  keep  its  equilibrium. 

There  are  two  classes  of  eyes  in  the  group:  (a)'  compound 
eyes,  made  up  of  numerous  similar  elements,  as 
in  the  insects  and  Crustacea,  and  (b)  simple 
eyes  —  ocelli  —  found  alone  in  spiders  and  in  many 
larvae,  or  in  connection  with  the  compound  eyes, 
as  in  many  insects. 

The  compound  eye  is  made  up  of  elements 
(ommatidia)  radially  arranged  about  the  end  of 
the  optic  nerve.  Each  ommatidium  is  probably 
capable  of  forming  an  image  of  a  limited  portion  of 
the  field,  and  consists  of  (i)  a  cuticular  cornea, 
appearing  externally  as  a  "facet,"  (2)  a  cellular 
lens  or  cone  which  directs  the  rays  of  light,  (3) 
sensory  retinal  cells  which  receive  the  light,  and 


FIG.  127.  An  ommatidium  or  eye-element  from  the  eye  of  the  Lobster 
(after  G.  H.  Parker),  c,  cornea  (cuticle);  c.h.,  corneal  hypodermis,  which 
secretes  the  cuticle;  co.,  cone  cells;  cr.,  crystalline  cone;  n,  nuclei;  n./.,  nerve 
fibres;  r.d.,  distal  or  outer  retinula  cells;  r.p.,  proximal  or  inner  retinula  cells; 
rh.,  rhabdome. 

Questions  on  the  figure.  —  Identify  the  following  regions: 

(1)  protecting  part  including  the  cornea  and  hypodermal  cells; 

(2)  focusing  portion,  —  the  crystalline  cone  and  the  cone  cells; 

(3)  the  pigmental  elements  of  the  retina  (distal  and  proximal 
retinular  cells),  the  former  of  which  prevent  rays  of  light  en- 
tering one  ommatidium  from  passing  obliquely  into  adjacent 
ones;  the  proximal  cells  may  be  more  immediately  connected 
with   (4)  the  nervous  elements  which  unite  the  eye  with  the 
nerve  centres.     Define  an  ommatidium.     Is  it  known  whether 
the  image  is  inverted  in  such  an  eye  as  this? 


(4)  pigment  cells  which  separate  the  retinal  elements  of  adjacent 
ommatidia,  and  play  an  important,  though  not  fully  understood, 
r61e  in  vision  (see  Figs.  44  and  127). 

326.  Library  Exercise.  —  If  time  allows  some  student  might  be  required  to  make 
a  more  detailed  report  of  the  structure  of  the  compound  eye  in  Arthropods  and  its 
method  of  image  formation.  Other  reports  may  be  made,  in  which  drawings  of 
the  various  sense-organs  in  arthropods  are  presented  to  the  class,  especially  the 
various  types  of  auditory  organs. 


286  ZOOLOGY 

327.  Reproduction  and  the  Reproductive  Organs. — Repro- 
duction  in    arthropods   is   sexual.     With   few   exceptions   the 
sexes  are  permanently  separate.     There  is  often  much  differ- 
ence in  the  size,  color,  structure,  and  activity  of  the  two  sexes. 
The  males  are  often  smaller,   more  active,   and  more  highly 
colored  than  the  females   (see    "sexual  dimorphism,"    §149). 
Sometimes  the  members  of  a  single  sex  are  dimorphic,  as  in 
the  workers  and  queens  among  the  bees.     This  is  correlated 
with  individual  division  of  labor  in  the  social  insects. 

The  sexual  organs  are  usually  paired,  and  in  the  female 
consist  of  the  ovaries  (which  may  be  subdivided  into  ovarioles) , 
oviducts,  receptacula  seminis,  in  which  spermatozoa  are  stored 
at  copulation,  accessory  glands,  sometimes  external  copulating 
and  egg-depositing  organs.  The  male  has  testes;  vasa  deferentia, 
which  may  have  special  enlargements  for  the  storing  of  sper- 
matozoa and  the  formation  of  sperm  masses ;  and  external  copu- 
latory  organs.  See  figures  of  the  sexual  organs  of  the  honey-bee 
or  other  representative  insect  in  the  reference  texts.  Compare 
them  with  those  of  the  snail. 

328.  Parthenogenesis. — In   several   insect   types   the   eggs 
have  the  power  of  developing  without  being  fertilized  by  the 
male  element.     Its  occurrence  is  determined  primarily  by  the 
absence  of  males,  but  even  when  males  are  present  the  female 
may  often  deposit  unfertilized  eggs.     She  is  influenced  to  do 
this  possibly  by  the  special  conditions  of  temperature,  nutrition, 
and  the  like,  to  which  she  is  subject. 

The  individuals  resulting  from  parthenogenesis  may  differ 
very  materially  from  those  produced  by  the  normal  sexual 
method.  In  the  case  of  the  bee,  the  males  or  drones  come  from 
unfertilized  eggs,  and  the  workers  and  queens  from  fertilized. 
The  cause  of  the  differences  between  workers  and  queens  is 
apparently  one  of  nutrition  purely.  Biologically,  partheno- 
genesis is  to  be  considered  as  a  modified  or  abbreviated  form 
of  sexual  reproduction,  in  which  the  stimulus  to  cleavage  comes 
from  some  source  other  than  the  male  cell. 

329.  Development. — After  fertilization  the  nucleus  divides 
as  described  for  other  forms,  but  usually,  on  account  of  the 


ARTHROPODA 


287 


abundant  yolk  which  the  eggs  contain,  complete  segmentation 
of  the  cell  is  not  effected.  After  a  series  of  divisions  some  of 
the  free  nuclei  assume  a  superficial  position  where  they  become 
surrounded  by  protoplasm,  and  form  the  blastoderm  (Fig.  13, 
D,  3)..  This  is  described  as  partial  and  peripheral  segmenta- 
tion. On  the  side  of  the  egg  where  the  embryo  is  to  lie,  a 
thickening  called  the  ventral  plate  is  formed.  From  this  area 
of  the  blastoderm  there  arises,  by  specialization,  by  insinking, 
and  by  multiplication  of  the  cells,  the  three-layered  condition. 
The  presence  of  yolk  so  obscures  and  complicates  the  process 
that  the  student  must  be  referred,  to  more  comprehensive  books 
for  even  an  outline  of  it. 

330.  The  Later  Development  may  be  either  direct  or  in- 
direct.    That  is  to  say,  the  young  when  hatched  may  be  the 

FIG.  128. 


PIG.  128.     The  Z6ea  of  Cancer  irroratus.     From  Verrill.     X  15. 

Questions  on  the  figure. — Compare  with  the  larva  of  lobster  (Fig.  131)  and 
with  the  Megalops  (Fig.  129),  and  note  likenesses  and  differences. 

adult  in  miniature,  possessing  its  form  and  habits,  or  may 
have  a  very  different  form  and  assume  the  adult  condition  by 
one  or  more  metamorphoses.  The  differences  between  the  larval 
and  adult  conditions  may  be  slight  or  very  great.  To  effect 


288 


ZOOLOGY 


the  change  from  larva  to  ^adult  a  series  of  moultings  of  the 
chitinous  covering  is  usually  necessary;  these  may  be  accom- 
panied or  preceded  by  periods  of  rest,  in  which  important  in- 
ternal changes  take  place.  The  metamorphosis  is  more  common 
among  insects  (Figs.  142  and  148),  although  a  similar  thing 
happens  in  many  of  the  Crustacea  (as  crabs,  Figs.  128  to  130). 
In  spiders  the  development  is  direct.  The  eggs  of  many  insects 
hatch  as  worm-like  larvae  (grubs,  maggots,  caterpillars).  These 
are  usually  active,  voracious,  fat-storing  animals,  which  after 

FIG.  129. 


FIG.  129.     Megalops  of  Cancer  irroratus.     From  Verrill.     X  15. 

Questions  on  the  figure. — Compare  with  Pigs.  128  and  131,  and  make  note  of 
the  chief  points  of  contrast.  Compare  also  with  adult  crab  (Fig.  130).  What 
differences  are  to  be  noted  between  the  development  in  the  lobster  and  in  crabs? 
Is  the  larval  or  adult  crab  more  like  the  lobster? 

a  period  pass  into  a  resting  condition,  often  surrounding  them- 
selves with  protective  coverings  (cocoons) .  During  this  quiescent 
stage  they  are  described  as  pupcz.  In  the  pupal  stage  the  accu- 
mulated fat  is  used  by  the  organism  in  forming  the  new  organs 
of  the  adult  or  imago.  The  internal  larval  organs  may  be  torn 
down  completely  by  the  aid  of  amoeboid  cells  and  be  made  to 
contribute  material  to  rebuild  the  new.  The  extent  of  these 


ARTHROPODA  289 

changes  can  only  be  realized  by  a  comparison  of  the  structure 
of  a  caterpillar  and  of  the  butterfly  into  which  it  develops.  The 
larvae  may  be  suited  to  aquatic  life,  the  adult  to  aerial ;  the  larva 
may  be  carnivorous  or  herbivorous,  the  adult  may  live  on  the 
nectar  of  flowers.  These  changes  of  habit  are  closely  correlated 
with  the  changes  of  structure  noted  in  the  metamorphosis. 
The  reproductive  organs  are  not  mature  until  the  imago  stage 
is  reached.  Frequently  the  imago  only  survives  long  enough 
to  insure  the  laying  of  fertilized  eggs.  In  general  the  length  of 

FIG.  130. 


FIG.  130.     Violet  Land-crab.     After  Shufeldt. 

Questions  on  the  figure. — Compare  the  crab  with  the  lobster  (Fig.  132)  as  to 
the  development  of  the  body-regions,  segmentation,  appendages,  etc.  Compare 
all  the  figures  of  crabs  available  and  note  in  what  respects  they  vary  externally. 

331.  Library  References. — Make  a  report  on  the  metamorphosis  in  Crustacea. 
What  is  meant  by  an  incomplete  metamorphosis?  Illustrations. 

life  in  insects  is  short,  although  it  is  claimed  that  queen  bees 
may  live  five  years  and  queen  ants  thirteen.  The  Crustacea 
are  much  longer  lived. 

332.  Ecology. — When  we  remember  the  great  number  of 
species  and  of  individuals  in  the  group  of  arthropods  we  are 
forced  to  realize  something  of  their  importance  in  their  rela- 
tion to  other  forms  of  life  on  the  earth.  Their  numbers  and 
their  enormous  power  of  reproduction  make  it  inevitable  that 
they  become  pests  and  threaten  the  existence  of  the  plants  and 
animals  on  which  they  prey,  and  likewise  that  they  become 
19 


2QO  ZOOLOGY 

important  elements  in  the  food  supply  of  animals  which  prey 
on  them.  It  is  only  by  their  great  reproductive  power  that 
they  can  hold  their  own  against  their  many  enemies, — the  birds 
and  other  insectivorous  animals,  and  the  accidents  of  climate, 
etc.  The  Crustacea  furnish  a  large  part  of  the  food  of  fishes. 

From  a  human  point  of  view  they  may  be  the  greatest  pests 
or  the  greatest  helpers.  In  the  voracious  larval  stage  they 
devour  waste  material  as  scavengers,  strip  vegetation,  spread 
disease,  produce  silk,  and  furnish  food  to  the  higher  animals. 
In  the  adult  stage  they  may  destroy  crops;  cross-fertilize 
flowers  in  their  search  for  nectar,  which  they  may  store  for 
themselves  and  their  young — to  be  intercepted  in  the  case  of 
the  bee  by  man;  may  spread  contagious  diseases;  may  devour 
stored  grain,  or  by  their  mere  presence  become  a  nuisance  to 
man  and  the  domestic  animals.  In  both  stages  they  may  be 
parasites  on  man  and  other  animals.  Few  of  the  arthropods 
are  directly  useful  as  food  to  man,  though  lobsters,  crayfish, 
shrimps,  etc.,  are  important  items  in  our  food  supply. 

Many  special  devices  of  structure  and  of  instinct  have  arisen 
making  their  continued  existence  in  the  presence  of  their  enemies 
possible.  Indeed  there  is  no  group  of  animals  in  which  so 
many  and  such  interesting  adaptations  to  the  special  conditions 
of  life  are  found  as  among  the  arthropods.  All  are  provided 
with  some  degree  of  external  protective  covering.  Many  are 
so  colored  and  shaped  as  to  be  inconspicuous  in  their  natural 
environment.  Some  are  endowed  with  offensive  odor  and 
taste,  some  with  stinging  organs.  Others  which  are  them- 
selves perfectly  harmless  are  so  much  like  forms  which  are 
repulsive  or  dangerous,  as  to  be  preserved  thereby  from  their 
enemies  (see  Chapter  VIII). 

Many  insects  as  ants,  bees,  and  wasps  are  strikingly  social 
in  their  habits,  and  show  a  high  degree  of  differentiation  among 
themselves.  Among  the  bees  a  special  class  of  females — the 
queens — lay  the  fertilized  eggs,  the  other  females — the  workers 
— being  sexually  immature.  In  the  ants,  still  further  division 
of  labor  occurs  among  the  workers.  Some  individuals  act  as 
soldiers  for  the  protection  of  the  ordinary  workers.  Some 
species  of  ants  make  slaves  of  other  species  of  ants  which  do  the 


ARTHROPODA  2QI 

work  of  the  colony,  or  of  other  animals  (aphides)  for  the  purpose 
of  feeding  on  their  secretions.  A  high  order  of  social  instincts 
and  skill  is  shown  by  certain  members  of  this  group, — the 
highest,  apparently,  shown  by  any  of  the  invertebrate  animals. 

333.  Behavior. — In  the  Arthropods  we  are  dealing  with 
organisms  more  complex  in  their  nervous  organization  and  with 
a  greater  variety  of  sense  organs  than  we  have  yet  found, — 
with  the  possible  exception  of  some  of  the  cephalopods.  Their 
reactions  to  stimuli  and  general  behavior  in  the  presence  of  the 
conditions  of  life,  and  probably  their  mental  life,  are  correspond- 
ingly rich  and  varied.  We  feel  that  their  activities  are  reflex 
and  instinctive  rather  than  intelligent,  although  it  has  been 
definitely  shown  by  experiment  that  the  crayfish  and  some  of 
the  insects  learn  to  accomplish  things  by  the  trial  and  error 
method.  That  is,  they  modify  behavior  through  experience. 

The  nervous  system,  scattered  as  it  is  largely  through  the 
segments  of  the  body,  is  suited  more  to  reflex  activities  than  to 
intelligent  activities.  The  latter  become  more  pronounced  as 
the  brain  comes  to  contain  a  larger  portion  of  the  nervous 
matter,  with  more  complex  connections. 

Crayfish  prefer  darkness  and  are  positively  thigmotropic 
(that  is,  like  to  touch  solid  objects).  Both  these  reactions  are 
valuable  in  bringing  them  into  hiding. 

Some  species  of  insects  show  striking  powers  of  recognizing 
one  another,  and  of  communicating  their  mental  states.  This  is 
done  apparently  through  tactile  and  chemical  sense  organs  in 
the  antennae  and  other  appendages  and  by  the  making  and 
hearing  of  sounds.  The  whirring  and  buzzing  of  wings  in  flies 
and  bees  and  mosquitoes,  the  scraping  of  legs  and  wings  by  which 
chirping  is  done,  the  sustained  stridulations  of  the  cicada  are 
examples  of  these.  The  lazy  hum  of  the  bee  is  very  different 
from  the  angry  buzz,  and  is  quickly  recognized  by  other  mem- 
bers of  the  hive.  In  others  as  the  mosquito  the  song  of  the 
female  is  probably  a  sex  call.  Probably  many  of  them,  such 
as  the  tappings  of  some  beetles,  are  without  meaning  except  as 
evidences  of  nervous  activity.  Others  are  merely  incidental  to 
necessary  activities,  as  flying. 


2Q2  ZOOLOGY 

The  complex  instincts  of  many  insects  as  yet  defy  our  anal- 
ysis. They  are  often  set  off,  so  to  speak,  by  external  conditions, 
as  the  laying  of  eggs  on  decaying  meat  by  flies;  but  we  do  not 
know  to  what  extent  they  are  controlled  by  external  and  to 
what  by  internal  conditions.  The  most  remarkable  instincts 
are  connected  with  the  place  and  time  of  egg  laying,  the  building 
of  homes,  the  capturing  of  prey,  and  the  complex  social  life  of 
the  higher  forms.  The  student  can  well  afford  to  seek  illustra- 
tions of  these. 

In  some  of  the  Crustacea  there  is  an  interesting  power  of 
breaking  off  an  injured  leg  at  a  definite  point.  This  is  not  because 
of  particular  weakness  at  this  point,  but  is  the  result  of  definite 
muscular  contractions.  It  is  a  reflex  and  instinctive  response 
to  certain  stimuli.  The  regeneration  of  the  leg  occurs  from  the 
stump.  Many  crustaceans  have  the  power  of  regenerating 
any  of  the  appendages  which  may  be  lost.  This  is  true  also  of 
the  eyes,  which  are  on  stalks.  Antennae,  however,  may  develop 
in  place  of  the  eyes. 

334.  Library  Exercises. — Reports  on  the  social  life  of  bees  and  ants;  on  the 
animals  captured  and  utilized  by  ants;  on  power  of  flight  in  ants;  on  queens  among 
ants  and  bees;  on  myrmecophilous  (ant-loving)  insects;  on  intelligence  among 
insects  and  spiders. 

335.  Classification. 

Class  I.  Crustacea  (with  shell;  Crayfish,  Crabs,  Barnacles,  etc.). — Arthropoda, 
chiefly  inhabiting  the  water  and  breathing  by  means  of  gills  or  through  the  body 
wall.  The  head  typically  consists  of  five  segments  more  or  less  fused  and  bearing 
two  pairs  of  antennae  or  feelers,  one  pair  of  mandibles,  and  two  pairs  of  maxillae. 
The  thorax  or  second  region  of  the  body  may  be  separate  from  or  fused  with  the 
head  (cephalothorax) .  It  possesses  a  variable  number  of  segments,  which  usually 
bear  the  locomotor  appendages.  The  remainder  of  the  body  (abdominal  segments) 
is  normally  of  distinct  segments  in  which  the  appendages  are  much  reduced.  The 
chitinous  skeleton  is  ordinarily  well  developed. 

Subclass  I.  Entomostraca. — Crustacea,  small  and  simple  in  organization,  with 
a  variable  number  of  segments  of  .which  the  appendages  are  simple  and  less  diverse 
than  in  the  next  subclass.  Many  of  them  are  parasitic  and  degenerate.  A  meta- 
morphosis occurs.  The  group  embraces  many  small  free  forms,  found  both  in 
fresh  and  salt  water,  some  fish  parasites,  and  the  sedentary  barnacles.  Here 
belong  Cyclops  and  Daphnia,  which  occur  abundantly  in  fresh- water  pools  and  feed 
on  the  algae  common  there.  They  constitute  an  important  portion  of  the  food  of 
the  fresh-water  fishes.  They  multiply  very  rapidly  and  keep  closely  up  to  the 
limit  of  the  food  supply.  The  eggs  of  many  of  them  can  resist  drying  to  a  re- 
markable degree.  This  is  of  manifest  importance  when  we  remember  that  they 
frequent  pools  in  which  drouth  is  not  uncommon. 


ARTHROPOD A  2 93 

The  barnacles  (Cirripedia)  are  Crustacea  which  in  adult  life  become  attached 
to  the  rocks  near' low  water-mark  or  to  floating  objects  of  various  kinds.  The 
bottoms  of  ships  become  foul  with  them.  The  group  is  especially  interesting  in 
that  it  points  to  the  giving  up  of  free  motion,  which  its  ancestors  possessed,  for  a 
mode  of  life  wholly  different,  and  demanding  marked  changes  of  structure.  They 
possess,  besides  the  organs  for  attaching  themselves,  bivalve  shells  similar  to  those 
of  Mollusca,  for  protection;  they  are  often  hermaphroditic,  which  is  a  very  un- 
common thing  in  arthopods.  The  advantages  gained  by  their  special  habits  are 
evident.  The  waters  near  the  shore  contain  a  great  deal  of  organic  de"bris,  and  any 
organism  which  can  attach  itself  here  and  yet  be  protected  from  destruction  by 
the  waves  is  fortunate.  Those  attached  to  floating  objects  are  brought,  without 
their  effort,  into  constantly  changing  localities. 

Subclass  II.  Malacostraca. — Crustacea  of  larger  size  and  more  highly  organized. 
Segments,  except  in  one  order,  twenty,  and  well  differentiated.  Nineteen  of 
these  segments  bear  appendages.  The  first  stage  in  the  metamorphosis  (the 

PIG.  131. 


FIG.   131.     Larva  of  Lobster   (Homarus  americanus)   removed  from   egg  shell.     From   Herrick. 

Questions  on  the  figure. — Compare  with  the  adult  (Fig.  132)  and  note  similari- 
ties and  differences?  Examine  Dr.  Herrick's  figures  (Bull.  U.  S.  Fish  Commission 
for  1895)  and  notice  the  gradual  change  to  the  adult  condition  by  successive 
moultings.  What  structures  can  you  identify? 

nauplius)  is  usually  passed  before  hatching.  The  group  embraces  (i)  the  Deca- 
poda,  or  the  lobsters,  crabs,  crayfishes  and  shrimps,  which  agree  in  the  possession 
of  ten  walking  feet,  eyes  on  movable  stalks  and  a  carapace  covering  the  thirteen 
fused  segments  of  the  cephalothorax ;  and  (2)  the  Tetradecappda,  comprising 
numerous  smaller  types  such  as  beach-fleas,  sow-bugs  or  wood-lice,  in  which  head 
and  thorax  are  not  fused,  the  eyes  are  not  movable,  and  the  walking  appendages 
are  fourteen. 

The  crayfish  and  lobsters  have  well-developed  abdominal  segments,  whereas 
in  the  crabs  the  abdomen  is  reduced  and  bent  under  the  thorax,  which  becomes 
broad  and  massive  (Figs.  129,  130).  The  larger  Crustacea  are  omnivorous,  almost 
all  organic  matter,  dead  or  living,  being  acceptable.  Lobsters  are  known  to  attack 
and  devour  fishes.  The  lobster  (Homarus,  Figs.  131  and  132),  of  which  there  are 
two  species, — an  American  and  a  European, — is  economically  the  most  important 
member  of  the  group,  and  stands  next  the  oyster  as  the  most  important  invertebrate 


294 


ZOOLOGY 
FlG.   132. 


FIG.  132.     The  American  Lobster  (Homarus  americanus) .     From  Herrick. 

Questions  on  the  figure. — What  body-regions  are  distinguishable  in  the 
lobster?  Compare  by  actual  measurement  the  size  of  the  crushing  claw  with  that 
of  the  body.  How  many  segments  in  the  abdominal  region?  Compare  with 
Fig.  130. 

food  species.  It  is  estimated  that  as  many  as  one  hundred  million  lobsters  have 
been  taken  in  a  single  year  in  New  England  and  Canadian  waters.  There  is  no 
doubt  that  the  lobster  is  in  immediate  danger  of  extinction  as  a  food  animal,  as  is 
shown  by  the  fact  of  greater  difficulty  in  obtaining  them  and  by  the  decrease  in 
the  average  size  of  the  animals  put  on  the  market.  This  decrease  occurs  in  the 
face  of  the  fact  that  the  mature  female  produces  from  ten  thousand  to  one  hundred 
thousand  eggs.  These  are  carried  under  the  abdomen  of  the  mother  until  hatched, 
which  requires  a  period  of  ten  or  eleven  months.  After  hatching  the  young 
undergo  a  series  of  moultings  during  which  time  they  are  the  prey  of  many  kinds 
of  enemies.  Such  is  the  mortality  that,  on  an  average,  not  so  many  as  two  of  all 


ARTHROPODA  2Q5 

the  young  of  a  female  reach  maturity.  This  is  another  way  of  saying  that 
a  species  is  losing  ground.  Two  general  methods  have  been  tried  to  make  good  the 
decline  in  the  supply:  first,  legislation  forbidding  the  taking  of  animals  under  the 
size,  which  indicates  sexual  maturity  (eight  to  twelve  inches),  and  forbidding 

FIG.  133. 


FIG.  133.     Palcemonetes  vulgaris.     From  Verrill. 

Questions  on  the  figure. — Compare  the  appendages  of  Palaemonetes  with  those 
of  the  lobster,  the  crab  and  Gammarus.  What  seem  to  be  the  functions  of  the 
various  appendages,  so  far  as  position  and  form  may  indicate? 

FIG.  134. 


FIG.  134.     Gammarus  ornatus.     From  Verrill. 

Questions  on  the  figure. — How  does  this  form  compare  with  the  lobster  and 
the  crabs  in  differentiation  of  the  segments,  in  fusion  of  the  segments  and  in  the 
differentiation  of  the  appendages? 

the  capture  of  females  carrying  the  developing  embryos;  and,  second,  attempts  on 
the  part  of  the  national  government  to  hatch  artificially  and  care  for  the  moulting 
young  under  such  conditions  that  they  will  be  protected  from  their  natural  enemies. 
The  problem  is  not  yet  solved,  and  in  the  meantime  another  source  of  food  is  likely 
to  be  destroyed  through  overfishing. 

The  crayfish  is  prized  for  food  in  European  countries,  but  is  little  used  in 
America  as  yet.  Shrimps,  prawns,  the  "soft-shelled"  or  blue  crab  are  all  of 
considerable  importance  in  this  regard.  The  smaller  Crustacea  are  a  very  im- 
portant element  in  the  food  supply  of  the  fishes,  both  in  the  fresh  waters  and  in  the 
sea. 


296 


ZOOLOGY 


Class  II.  Onychophora. — Peripattis  is  the  best  known  genus  of  this  class.  It 
is  interesting  chiefly  because  it  is,  in  some  degree,  intermediate  between  the 
Annulata  and  the  higher  arthropods.  They  are  remarkable  for  a  wide  distribu- 
tion out  of  proportion  to  their  numbers,  and  are  found  in  moist  places,  under 
wood,  stones,  and  in  rotting  bark.  They  agree  with  the  chaetopod  annulates 
(see  §276)  in  the  possession  of  segmental  organs  (nephridia),  a  dermo-muscular 
sac,  and  poorly  developed  appendages.  The  segments  are  also  homonomous 
(see  §264)  as  in  the  worms.  The  relationship  to  arthropods  is  indicated  by  the 
possession  of  tracheae,  by  the  substitution  of  haemocoele  (the  enlarged  lacunae  in 

FIG.  135. 


FIG.   135.     Caprella  geometrlca.     From  Verrill.      X  4. 

Questions  on  the  figure. — In  comparison  with  other  Crustacea  what  are  the 
aberrant  or  peculiar  features  of  this  form?  See  also  figures  in  reference  texts 
(e.g.,  Parker  and  HaswelTs  Zoology,  Vol.  I,  p.  546). 

which  circulation  occurs)  for  the  true  coelom,  and  by  the  differentiation  of  some  of 
the  anterior  segmental  appendages  as  mouth  parts.  The  Onychophora  resemble 
the  larval  condition  of  those  insects  which  undergo  a  metamorphosis  much  more 
than  the  adult  stages.  This  suggests  that  they  are  more  closely  related  to  the 
ancestral  types  from  which  the  insects  have  sprung  than  to  the  insects  themselves 
(Fig.  136). 

Class  III.  Myriapoda  (many  feet;  Centipedes,  etc.). — Tracheate  arthropods 
with*a  worm-like  body.  Segments  numerous,  and  much  alike,  one  (or,  in  Dip- 


FIG.  136. 


FIG.  136.     Peripatus  capensis.     From  Nicholson  after  Moseley. 

Questions  on  the  figure. — Externally  in  what  respects  is  this  form  like  the  An- 
nelids ?  In  what  respects  different  from  them  ?  Of  what  special  zoological  interest 
is  this  genus?  What  are  its  habits?  In  what  respects  is  it  like  and  in  what  unlike 
the  centipede  (Fig.  137)? 

lopoda,  two)  pair  of  appendages  to  each  segment.  The  head  is  distinct  and  bears 
antennae  and  mouth  parts.  The  eyes  are  numerous  and  simple  (ocelli).  In 
fundamental  structure  and  development  the  myriapods  resemble  insects.  There 
are  two  principal  orders.  One  embraces  the  centipedes  which  are  carnivorous, 
have  biting  jaws,  have  one  pair  of  appendages  to  each  segment,  and  are  poisonous. 
The  second  includes  the  millipedes  which  are  vegetable  feeders  and  possess  man- 


ARTHROPOD  A  2Q7 

dibles  suited  to  chewing  vegetable  matter.  They  are  wholly  harmless.  They 
have  two  pairs  of  legs  to  each  of  the  numerous  segments  except  the  first  four. 
Both  centipedes  and  millipedes  inhabit  the  land,  and  frequent  dark  places.  Many 
are  nocturnal  in  habit  (Fig.  137). 

Class  IV.  Hexapoda  (six  feet;  Insects). — Tracheate  arthropods  with  three  dis- 
tinct body  regions, — head,  thorax,  and  abdomen.  The  head  has  four  segments 
with  appendages, — a  pair  of  antennae  and  three  pairs  of  mouth  parts.  The  thorax 
has  three  segments  (pro-,  meso-  and  meta-thorax),  each  of  which  bears  a  pair  of 
legs;  the  meso-thorax  and  the  meta-thorax  may  each  bear  a  pair  of  wings.  The 
abdomen  has  a  variable  (often  obscure)  number  of  segments.  Its  appendages  are 
usually  entirely  wanting  or  much  reduced.  A  metamorphosis  frequently  occurs. 
The  larval  condition  often  suggests  the  annelids  and  the  myriapods  in  the  similarity 
of  its  segments,  and  in  the  numerous  appendages. 

The  student  is  referred  to  more  comprehensive  works  for  an  exposition  of  the 
numerous  orders  of  this  enormous  group  of  Hexapoda.  Only  the  more  important 
are  suggested  below. 

Fig.  137. 


\ 

FlG.  137.     Centipede  (Scolopendra  heros).     Photo  by  Folsom.     Four-fifths  natural  size. 

Questions  on  the  figure. — What  differentiation  of  segments  is  apparent?  Are 
there  any  fusions  into  body-regions?  What  is  the  law  of  the  occurrence  of  ap- 
pendages? What  diversity  is  there  among  them? 

Order  Aptera  (without  wings}. — This  order  embraces  a  number  of  minute, 
wingless  insects  which  do  not  undergo  any  metamorphosis.  The  body  is  covered 
with  scales  or  hairs.  The  spring-tails  and  snow-fleas  are  examples.  These  make 
their  leaps  by  suddenly  straightening  out  a  tail-like  structure  which  is  bent  under 
the  body  when  at  rest.  They  are  not  the  only  wingless  insects  and  hence  the 
name  is  somewhat  misleading.  See  Fig.  138. 

Order  Orthoptera  (straight  wings}. — In  this  order  the  metamorphosis  is  incom- 
plete or  lacking.  There  are  usually  two  pairs  of  wings,  the  anterior  often  some- 
what thickened,  serving  as  a  cover  for  the  posterior.  Mouth  parts  are  adapted 
to  biting  and  chewing.  Here  belong  the  cockroaches,  grasshoppers,  crickets, 
locusts,  katydids,  walking-stick  insect.  The  order  is  of  considerable  economic 
importance.  Most  of  its  members  are  vegetable  feeders  and  when  they  are 
gregarious  are  often  very  destructive.  The  Rocky  Mountain  locust,  so  named 
because  it  breeds  on  the  plateau  at  the  eastern  base  of  these  mountains,  in  1873 
and  again  in  1878,  migrated  eastward  over  Nebraska  and  Kansas  in  search  of  food, 
literally  stripping  fields  of  vegetation.  Since  the  settlement  of  the  regions  where 


298 


ZOOLOGY 


they  breed,  with  the  ploughing  up  of  the  eggs  and  the  destruction  of  the  young, 
there  is  reason  to  hope  that  these  migrations  are  at  an  end.  Accounts  of  similar 
migrations  of  locusts  are  recorded  in  the  history  of  the  old  world.  These  migra- 
tions and  their  effects  illustrate  how  climatic  conditions  in  one  locality  may  change 
the  balance  of  life  in  another.  The  second  chapter  of  the  prophet  Joel  gives  a 
vivid  account  of  a  visitation  of  locusts.  See  Fig.  139. 

FIG.  138. 


FIG.  138.     Campodea, — a   Thysanuran.     Magnified   30   times.     By  J.   W.   Folsom. 

Questions  on  the  figure. — In  what  respects  does  this  form  seem  intermediate 
between  the  Myriapods  and  the  higher  insects?  How  does  this  compare  with  the 
larvae  of  insects  which  undergo  a  metamorphosis?  Can  you  distinguish  head, 
thorax,  and  abdomen? 


Order  Neuroptera  (nerve-wings}. — The  members  of  this  order  have  a  more  or 
less  complete  metamorphosis.  Usually  there  are  two  pairs  of  netted  membranous 
wings.  The  mouth  parts  are  suited  to  biting.  This  group  includes  many  diverse 
forms  which  the  entomologists  distribute  into  nine  or  ten  orders.  These  are: 
the  Ephemerida,  or  may  flies  whose  adults  may  live  for  only  a  few  hours,  although 
the  larvae  may  require  from  one  to  three  years  to  develop;  the  Odonata,  or  carnivo- 
rous dragon-flies,  and  damsel-flies;  the  Plecoptera,  or  stone-flies;  the  Isoptera,  or 
termites  (white  ants)  which  are  social,  polymorphic  forms;  the  Corrodentia,  book- 


ARTHROPODA 


200 


lice  and  bark-lice;  the  Mallophaga,  parasitic  bird-lice  which  eat  hair  and  feathers; 
the  Physopoda,  or  thrips;  the  Dermaptera  or  earwigs;  the  Neuroptera,  the  Dobson 
flies,  ant-lions,  and  lace- winged-flies;  the  Trichoptera  or  caddice  flies,  which  in  the 
larval  stage  cements  around  itself  a  protecting  case  made  of  sand  or  other  particles 
of  matter. 

FIG.  139. 


FIG.   139.     Katydid   (Cyrtophyllus   perspicitlatus),   natural   size.     Photo   by  Folsom. 

Questions  on  the  figure. — How  many  pairs  of  appendages  are  visible  in  the 
figure?  How  many  pairs  are  present ?  To  what  order  of  insects  does  the  Katydid 
belong?  What  are  its  feeding  habits ?  What  can  you  find  of  its  development? 

FIG.  140. 


FIG.   140.     Periodical  Cicada.     Natural  size.     Photo  by  Folsom. 

Questions  on  the  figure. — To  which  order  of  insects  does  Cicada  belong? 
Which  -of  its  habits  are  most  familiar  to  you?  What  are  its  nearest  relatives 
among  the  insects? 

Order  Hemiptera  (half -wing}. — Hexapoda  with  an  incomplete  metamorphosis, 
and  having  two  pairs  of  wings,  or  none.  Mouth  parts  are  modified  for  piercing 
and  sucking.  Here  are  included  the  true  bugs,  as  the  squash  bug,  chinch  bug,  bed- 
bug, the  water  boatman,  etc.;  the  lice;  the  plant-lice;  the  scale  insects ;  and  the 
cicadas  (sometimes  called  "locusts").  These  should  not  be  confused  with  the 


300 


ZOOLOGY 


beetles,  which  are  often  called  "bugs."     See  Fig.  140.     The  Hemiptera  furnish 
some  really  serious  pests,  as  the  scale  insects,  aphids,  chinch  bugs,  etc. 

Order  Diptera  (two  wings}. — These  Hexapoda  undergo  a  complete  metamor- 
phosis, having  the  anterior  pair  of  wings  developed  (not  in  fleas).  The  posterior 
pair  is  very  much  reduced  or  wanting.  The  mouth  parts  are  well  adapted  for 
piercing  and  sucking.  The  order  is  very  large  in  species  and  includes  such  common 
forms  as  the  flies,  mosquitoes,  gnats,  fleas.  Many  members  of  this  group  are  of 
great  importance  to  man.  The  maggots  of  the  true  flies  are  scavengers,  developing 
in  decaying  organic  matter  and  assisting  in  its  destruction;  the  adults,  on  the  other 
hand,  besides  being  unpleasant  companions  and  demanding  a  share  of  our  com- 

FIG.  141. 


FIG.   141.     Larv ae  of  the  Bot-fly  (Gastrophilus  equf)  in  the  stomach  of  the  horse.     One-half  natural 
size.     From  Luggar,  after  Heller. 

Questions  on  the  figure. — What  do  you 'know  of  the  habits  of  the  bot-fly? 
Where  are  the  eggs  deposited?  How  do  the  larvae  come  to  have  the  position 
figured  above?  How  do  they  pass  from  this  to  the  adult  condition?  See  also 
Fig.  142.  How  does  it  retain  its  position  in  the  stomach  of  its  host? 


forts,  spread  disease.  Other  species  suck  the  blood  of  man  and  domestic  animals, 
producing  disease  and  death.  The  bot-flies  are  most  important  in  their  larval 
stage.  The  eggs,  deposited  on  the  exterior,  are  taken  into  the  digestive  tract  and 
there  develop,  often  migrating  into  other  organs  and  producing  definite  diseases. 
Mosquito  larvae  devour  the  decaying  organic  matter  in  stagnant  pools.  The  adult, 
especially  the  female,  is  a  blood-sucker  and  is,  through  the  parasitic  protozoa 
which  may  infest  it,  the  chief  instrument  of  the  spread  of  malaria  and  yellow  fever 
among  men.  They  are  all  very  prolific  and  develop  rapidly  considering  the  fact 
that  they  undergo  a  metamorphosis.  The  fly,  for  example  only  requires  a  few 


ARTHROPODA 


3OI 


hours  for  hatching  into  the  maggot  stage.  If  food  and  temperature  are  favorable, 
this  maggot  may  grow  to  full  size  in  a  week,  when  it  passes  into  the  resting  or  pupa 
stage,  from  which  another  week  or  more  is  required  for  the  young  fly  to  emerge. 

The  eggs  of  mosquitoes  are  deposited  in  water,  where  they  hatch  into  active 
larvae  called  "  wigglers."  These  breathe  the  air  by  means  of  a  tube  on  the  next  to 
the  last  abdominal  segment.  Their  common  position  with  the  end  of  the  tail  at 
the  surface  of  the  water  is  thus  explained.  The  mosquito  larva  does  not  cease 
to  be  active,  but  by  a  series  of  moults  comes  to  the  so-called  pupa  stage  from  which 

FIG.  142. 


PIG.  142.  Stages  in  the  development  of  the  Bot-fly  (Gastrophilus  equi).  From  Parker  and 
rfaswell,  after  Brehm.  a,  adult  insect;  b,  egg  attached  to  a  hair;  c.d.  and  e,  stages  in^he  development 
of  the  larva.  (See  also  Fig.  141.) 


by  an  early  moulting  the  adult  mosquito  emerges,  balancing  itself  on  the  floating 
pupal  skin  until  its  wings  are  hardened  sufficiently  for  use.  See  Fig.  143. 

The  Hessian-fly  deposits  its  eggs  in  the  tissues  of  growing  wheat  and  corn;  the 
clover-gnat  and  others  produce  galls  which  interfere  with  the  growth  of  the  plant, 
often  very  seriously.  In  the  case  of  the  Hessian-fly  great  damage  to  the  wheat 
crop  often  results.  See  Fig.  144. 

The  fleas  are  to  be  looked  upon  as  degenerate.  They  are  often  placed  in  a 
separate  order  (Siphonaptera) .  The  adults  are  external  parasites  without  wings. 
They  are  flattened  laterally  and  thus  pass  readily  between  the  hairs  of  the  host. 
The  larvae  are  not  parasitic,  but  live  on  decaying  organic  matter. 


302 


ZOOLOGY 


Order  Lepidoptera  (scale-wings}. — These  are  Hexapoda  which  pass  through  a 
complete  metamorphosis,  in  the  adult  possess  sucking  mouth  parts,  and  have  two 
pairs  of  large  membranous  wings  covered  with  scales.  The  moths  and  butterflies 
are  the  representatives  of  the  order.  The  larvae  are  known  as  caterpillars,  which, 
with  a  few  exceptions,  are  vegetable  feeders.  The  adult  butterfly  differs  from  the 


PIG. 


143.     Two  stages  in  the  metamorphosis  of  the   Mosquito.     From  Packard      A,  larva;  B, 
pupa;  C,  ventral  view  of  the  oar-like  appendages  of  the  last  segment  of  the  pupa. 


FIG.  144. 


PIG.  144. 


The  Hessian  Fly  (Cecidiomya  destructor).     From  Standard  Natural  History, 
adult;  b,  larva;  c,  pupa;  d,  larvae  in  position  on  stalk  of  wheat. 


a,  the 


Questions  on  the  figures. — Give  names  to  all  the  structures  apparent  on  the 
adult.  In  which  stage  does  the  insect  do  its  damage?  What  is  its  economic 
importance?  What  is  the  origin  of  its  common  name? 


moths  (typically)  in  the  fact  that  the  former  fly  by  day,  hold  the  wings  erect  when 
at  rest  and  have  antennae  with  a  club  on  the  end.  The  butterflies  share  with  the 
birds  the  preeminence  in  beauty  among  animals.  They  present  many  points  of 
interest  in  their  metamorphosis,  in  their  habits,  their  coloration  their  distribution, 
and  their  economic  effects. 


ARTHROPODA 


303 


The  caterpillars  are  usually  voracious  and  may  strip  their  food  plant  of  its 
leaves  and  buds.  The  majority  of  the  larvae  have  become  highly  specialized  in 
their  food  habits,  becoming  restricted  in  some  instances  to  one  species  or  to  a  few 
related  species  (as  illustrated  by  the  tomato  worm,  which  feeds  on  tomato,  potato, 

FIG.  145. 


C 


FIG.   145.     The  Cabbage  Worm  (Pieris  rapcz).     Natural  size.     Photo  by  Folsom.     A  and  B,  larvae; 

C,  pupa. 

*".  Questions  on  the  figure. — What  is  a  larva?  What  is  a  pupa?  Which  is  the 
earlier  stage?  What  is  the  color  of  this  caterpillar  in  nature?  See  the  next 
figure  for  the  adult. 

FIG.  146. 


FIG.  146.     The  adult  Cabbage  Butterfly  (Pieris  rapce).     Natural  size.     Photo  by  Folsom. 

Questions  on  the  figure. — Why  is  the  larva  of  this  animal  called  the  cabbage 
worm?  Why  is  the  adult  called  the  cabbage  butterfly?  What  are  its  feeding 
habits? 


and  tobacco  leaves;  or  the  cabbage  worm  which  eats  the  leaves  of  certain  of  the 
cruciferous  plants).  The  distribution  of  such  species  is  thus  clearly  determined 
by  that  of  their  host  plants.  The  most  injurious  to  vegetation  are  the  "tent- 
caterpillars"  which  occur  gregariously  and  spin  a  web-like  nest;  the  army- worm 
so-called  because  it  appears  and  moves  from  its  hatching  grounds  in  great  numbers; 


3°4 


ZOOLOGY 


the  cotton-boll  worm;  the  canker-worms  and  fruit-borers.  The  silk- worm  seems 
to  be  the  only  useful  member  of  the  order.  The  clothes-moth  lays  its  eggs  in 
woolens  or  furs,  its  larvae  thus  being  exceptional  in  preferring  animal  diet. 

The  adults  are  usually  short-lived  and  some  do  not  eat  at  all.  The  majority 
of  them  suck  nectar  from  flowers  and  juices  from  ripe  fruits  and  other  objects  by 
means  of  special  tubular  mouth  parts  which  are  modified  paired  appendages. 
They  carry  pollen  from  flower  to  flower,  effecting  cross-fertilization,  in  some 
instances.  The  color  of  the  larvae  and  the  adults  is  very  varied  and  has  close 
relation  to  the  environment  and  habits  of  the  animals.  We  have  already  noticed 
in  the  chapter  on  adaptations  (Chap.  VII)  how  the  coloration  may  be  protective. 
This  is  the  more  needed  since  the  group  has  many  enemies,  especially  in  the  larval 
stage.  The  power  of  reproduction  is  great.  Several  broods  per  year  may  be 
produced.  There  are  25,000  known  species  of  Lepidoptera,  7,000  of  which  occur 
in  North  America,  north  of  Mexico.  The  species  are  more  numerous  and  striking 
in  the  tropical  regions  of  South  America. 

FIG.  147. 


PIG.   147.     The  Army  Worm  (Leucania  unipuncta).     After  Riley.     A,  caterpillar;  B,  adult  moth. 

Questions  on  the  figures. — What  are  the  principal  facts  concerning  the  habits 
and  economic  importance  of  the  army- worm?  Why  is  it  so  called? 

Order  Coleoptera  (shield-wings}. — In  this  group  there  is  a  complete  meta- 
morphosis. The  mouth  parts  are  suited  to  biting  and  chewing.  The  front  wings 
(elytra)  are  hardened  and  serve  as  covers  for  the  true  membranous  wings  when  the 
latter  are  not  in  use.  These  are  the  beetles, — falsely  called  "bugs."  Although  a 
well-defined  order  the  beetles  are  very  various,  as  will  be  seen  from  the  fact  that 
there  have  been  described  over  eleven  thousand  species  for  this  continent  north  of 
Mexico.  There  are  said  to  be  more  than  one  hundred  thousand  known  species  of 
beetles. 

The  larvae  are  commonly  known  as  grubs.  The  feeding  habits  are  almost  as 
diversified  as  the  form.  Many  are  scavengers  and  lay  their  eggs  in  carrion  and 
other  decaying  matter ;  others  bore  into  wood  and  bark,  as  the  long-horned  beetles ; 
some  frequent  grain,  nuts,  fruits;  others  are  leaf-eaters;  a  few  devour  other  insects. 
The  Colorado  potato-beetle,  the  weevils,  the  museum  pest,  the  locust-borer  or 
the  hickory-borer  will  serve  to  illustrate  some  of  the  more  hurtful  representatives 
of  this  immense  order. 

Some  especially  interesting  forms  are  the  fire-flies,  the  scarabeids,  including 
the  sacred  dung-beetle  of  Egypt,  and  the  ladybird-beetle.  The  latter  is  useful  to 
man  owing  to  the  fact  that  it  preys  on  certain  hurtful  insects.  In  California  the 
cottony-cushion  scale,  which  in  some  way  had  been  imported  from  Australia, 


ARTHROPODA 


305 


promised  at  one  time  to  destroy  totally  the  orange  industry.  The  Australian 
ladybug,  which  keeps  it  within  bounds  in  its  native  home,  was  imported,  and  the 
increase  of  the  ladybugs  was  such  that  the  cushion-scales  were  all  but  destroyed. 
This  species  of  ladybug  feeds  exclusively  on  the  cottony-cushion  scale,  and  there- 
fore the  destruction  of  the  latter  led  in  turn  to  the  rapid  decline  of  the  ladybugs 
from  the  loss  of  their  food  supply.  Indeed  it  was  necessary  to  keep  colonies  of 
the  scale  insects  protected  in  order  to  furnish  food  and  to  prevent  the  entire 
destruction  of  the  imported  beetle  by  starvation.  In  Australia  where  both  are  at 

FIG.  148. 


FIG.   148.     Swallow-tail  Butterfly  (Pafiilio  machaon), — larva,  pupa,  and  adult.     From  Nicholson. 

Questions  on  the  figure. — Which  is  the  larva  and  which  the  pupa?  Which 
of  these  is  the  earlier  stage?  What  are  the  chief  characteristics  of  the  three  stages 
in  the  metamorphosis  of  butterflies, — the  larva,  the  pupa  and  the  imago? 

home  the  natural  conditions  and  the  adjustment  of  the  two  species  are  such  that 
this  scale-insect  does  not  become  a  pest.  The  discovery  of  the  biological  relations 
of  these  species,  and  the  relief  of  the  orange  industry  furnish  a  sample  of  the  ex- 
cellent work  being  done  by  the  U.  S.  Department  of  Agriculture  in  connection  with 
the  economic  aspects  of  biology. 

Order  Hymenoptera  (membrane-wings}. — Hexapoda  with  four  membranous 
wings;  mouth  appendages  adapted  for  sucking  or  for  biting;  metamorphosis  com- 
plete. This  is  the  most  highly  developed  division  of  Insecta,  and  embraces  such 


306 


ZOOLOGY 


forms  as  bees,  wasps,  and  ants.  The  most  important  habits  of  the  group,  which 
are  those  growing  out  of  their  social  life,  have  been  referred  to  in  the  chapter  on 
adaptations  (Chapter  VIII).  The  chief  economic  value  of  the  order  is  in  the 
honey  of  the  honey-bee,  the  fertilization  by  bees  of  certain  plants,  as  clover,  and 
the  reduction  of  more  hurtful  species  of  insects  by  certain  parasitic  members  of 
the  order,  as  the  ichneumon-flies  and  chalcid-flies.  Some  of  the  larvae  are  leaf- 
eating,  as  the  rose-slug,  and  others  produce  galls  on  the  oak  and  other  plants  in 
depositing  their  eggs.  These  are  harmful  to  human  interests. 

FIG.  149. 


PIG.  149.     Hornet's  nest,  sectioned.     Photograph  from  life  by  Shufeldt. 

Class  V.  Arachnida  (spiders,  scorpions,  etc.). — Arachnida  are  arthropods  in 
which  the  head  and  thorax  are  typically  fused  and  represent  about  seven  segments 
with  six  pairs  of  appendages.  There  are  no  antennae.  The  abdomen  is  often 
segmented  but  usually  without  paired  appendages.  Respiratory  organs  are  con- 
fined to  the  abdomen,  and  may  be  of  three  types:  book-gills ,  associated  with 
appendages  (king-crab) ;  trachea  similar  to  those  of  insects;  and  book-lungs  (spiders). 
Development  is  usually  direct. 

Order  I.  Xiphosura  (the  king  crab). — This  order  contains  only  one  genus, 
Limulus,  a  marine  form  with  book-gills,  and  a  cuticular  test  like  that  of  the  Crus- 


ARTHROPODA 


307 


tacea,  with  which  it  was  formerly  classified.     Numerous  related  forms  flourished 
earlier  in  the  world's  history  but  are  now  extinct  (Trilobites). 

Order  II.  Scorpionida  (scorpions).  —  Arachnids  with  a  much  elongated  and 
segmented  abdominal  region  closely  connected  with  the  thorax.  They  are  air- 
breathers,  with  four  pairs  of  book-lungs  in  the  abdomen.  The  posterior  abdominal 
segments  form  a  tail  the  last  segment  of  which  bears  a  sting.  See  Fig.  150. 

FIG.  150. 


FIG.   150.     Scorpion  (Buthus).     Photo  by  Folsom. 

Questions  on  the  figure. — Compare  the  scorpion  with  figures  of  Crustacea, 
insects  and  spiders,  noting  the  chief  differences  and  likenesses.  Of  what  use  is 
the  long,  segmented  abdomen  in  the  scorpion? 

Order  III.  Araneida  (spiders). — The  Araneida  are  air-breathing  arachnids, 
with  book-lungs  alone  or  in  connection  with  tracheae.  Poison  glands  are  common 
in  connection  with  the  first  pair  of  (mouth)  appendages.  The  abdomen  is  un- 
segmented  and  without  appendages,  unless  the  spinnerets  represent  reduced 
appendages.  On  these  latter,  open  the  ducts  of  the  numerous  glands  secreting 
the  fluid  which  hardens  on  exposure  to  the  atmosphere  and  makes  the  silk  of  the 
web. 

Spiders  may  be  classified  on  the  basis  of  the  type  of  web  which  they  make. 
The  "orb-weavers"  construct  webs  of  great  regularity  and  beauty;  others,  as  the 


3c8 


ZOOLOGY 
FlG.   151. 


PIG.  151.     Spiders  (Epeira  marmorea).     After  McCook.     Male  on  left;  female  on  right.     Natural 

size. 

Questions  on  the  figure. — What  differences  do  you  note  with  respect  to  the 
sexes?  What  habits  of  the  spiders  are  correlated  with  this  difference  in  size  in  the 
sexes? 


FIG.  152. 


FIG.   152.     Web  of  Epeira  slrix,  an  Orb-weaving  Spider.     After  McCook. 

Questions  on  the  figure. — By  reference  to  other  texts  or  by  observation 
determine  if  there  is  any  regular  order  in  which  the  parts  of  the  web  are  produced. 
To  what  is  this  form  of  web  an  adaptation?  Evidences?  What  other  forms  of 
webs  are  constructed  for  similar  purposes? 


ARTHROPODA  309 

cob-web  spider,  make  a  complex  and  irregular  mesh-work  of  fibres  running  in  all 
directions;  others  spin  a  web  similar  to  the  last  with  the  exception  that  at  one  point 
it  is  continued  into  a  tube  into  which  the  spider  retreats  for  hiding.  The  webs  of 
these  spiders  are  for  the  purpose  of  catching  flies  and  other  insects  on  which  the 
animal  feeds.  The  trap-door  spiders  make  a  tunnel  in  the  ground  which  they 
line  with  their  secretion;  a  door  is  woven  which  is  so  covered  with  materials  like 
those  about  the  nest  that  its  presence  is  effectually  hidden.  A  considerable  number 
of  spiders  do  not  spin  proper  webs,  but  use  their  secretion  merely  in  forming  cocoons 
for  their  eggs,  or  in  binding  together  objects  to  make  a  home.  This  wonderful 
secretion  is  used  by  the  spider  in  many  other  ways  than  in  the  capture  of  prey  and 
the  making  of  a  nest.  By  means  of  it  some  of  the  spiders  make  a  near  approach  to 
flying.  A  spider  may  bridge  the  space  from  one  object  to  another  either  by  fasten- 
ing one  end  of  the  strand  and  hanging  at  the  other,  or  by  sitting  still,  he  may  allow 
the  free  end  to  float  out  until  it  becomes  attached.  In  some  cases  at  least  it  is 
known  that,  by  spinning  thus  loose  silk  in  abundance,  the  weight  of  the  spider 
may  be  readily  carried  by  the  action  of  the  wind  upon  his  silken  sails. 

The  chief  economic  importance  of  spiders  lies  in  their  habit  of  preying  on  various 
insects,  of  which  they  destroy  considerable  numbers. 

The  Arachnida  embraces  a  number  of  other  orders  including  less  important 
or  less  easily  observed  animals,  as  the  mites,  certain  ticks,  harvest-men  or  "daddy- 
long-legs,"  and  many  parasitic  or  otherwise  degenerate  forms. 

336.  Suggestive  Studies,  for  Field  and  Library. 

1 .  Dimorphism  and  polymorphism  in  insects. 

2.  Protective  adaptations  in  insects. 

3.  What  senses  seem  most  used  among  the  insects? 

4.  Report  on  observed  signs  of  intelligence  among  arthro- 
pods. 

5.  Is  there  any  evidence  of  power  of  communication  among 
the  social  insects,  as  the  ants  ? 

6.  Courtship  among  the  spiders. 

7.  Spiders'  webs:  form,  position,   efficiency,  mode  of  con- 
struction. 

8.  There  are  some  insects  which  have  wings  during  a  por- 
tion of  their  life  but  lose  them  later.     Investigate  the  condi- 
tions and  find  an  explanation. 

9.  Report  an  observed  instance  of  insects  pollinating  flowers 
(i.e.,   transferring    pollen    from    one  to  another).     How  is  it 
effected  ?     Why  does  the  insect  do  it  ?     Is  the  pollination  of 
flowers  by  insects  deemed  a  common  and  important  phenome- 
non by  botanists  ? 

10.  Can  you  find  any  recorded  instances  of  what  may  be 


3io 


ZOOLOGY 


called  symbiosis  (see  §160)  between  insects  and  other  organ- 
isms? 

1 1 .  Have  you  any  experimental  evidence  as  to  how  growth 
can  take  place  in  forms  with  a  firm  external  skeleton  such  as 
that  of  the  crayfish  ? 

12.-  Do  you  have  any  reason  for  thinking  that  a  metamor- 
phosis is  advantageous  to  any  of  the  Arthropoda  ?  Is  it  in  any 
respect  disadvantageous  ? 

13.  How  is  moulting  effected  ? 

14.  Habits  of  the  "hermit  crabs." 

15.  The  lobster  and  its  habits. 

1 6.  Silkworm  culture,  value  and  methods  of. 

17.  Insects  introduced  from  foreign  countries. 

1 8.  The  history  of  the  efforts  to  find  enemies  to  some  of  the 
more  important  noxious  insects. 

19.  Relation  of  insects  to  the  culture  of  figs. 

20.  The  structure  and  habits  of  the  king  crab  (Limulus). 
Why  is  it  not  to  be  classed  with  the  true  crabs  ? 


CHAPTER  XVIII 
PHYLUM  XII.— CHORDATA 

337.  This  phylum  includes,  beside  the  typical  Vertebrata 
to  be  described  in  later  chapters  (fishes,  amphibians,  reptiles, 
birds   and   mammals),    several   groups   of  much   more   simple 
organization.     These  latter  forms  may  be  included  under  the 
general  head  Protovertebrata,  not  because  they  all  show  close 
relationship  among  themselves,  but  because  of  their  primitive 
character,   considered  as  Chordata.     They  are  of  very  great 
interest  to  the  biologist   on    account  of  the  hints   they  may 
offer  concerning  the  ancestors  of  the  vertebrates.     For  detailed 
description  of  the  manner  of  life  and  the  structure  of  these 
primitive  chordates  the  student  must  be  referred  to  advanced 
text-books  of  zoology. 

338.  General  Characters  of  the  Chordata  (Protovertebrata 
and  Vertebrata). — The  Protovertebrata   are   allied   with   the 
typical  vertebrates  and  separated  from  the  invertebrates  by 
the  possession,  either  in  the  larval  or  adult  condition,  of  the 
following  features : 

1.  A  mid-dorsal,   longitudinal  rod  of  cells   (noto chord)   de- 
rived from  the  entoderm,  but  often  surrounded  by  mesodermal 
structures  (see  Fig.  157).     This  lies  ventral  to  and  supports 

2.  The  central  nervous  system,  a  mid-dorsal  cellular  tube 
with  thickened  walls  derived  from  the  ectoderm. 

3.  Gill-slits  or  perforations  connect  the  cavity  of  the  pharynx 
with  the  outside  directly  or  through  an  atrial  chamber. 

4.  The  heart  is  typically  ventral  to  the  digestive  tract. 

339.  In  the  Group  of  Protovertebrates  may  be  placed: 

i.  Balanoglossus,  a  soft-bodied,  worm-like  form  whose  claim 
to  a  place  among  the  Chordata  rests  upon  the  fact  that  an  out- 
growth of  the  gut  extends  into  the  proboscis,  where  it  forms  a 
solid  rod  which  in  its  origin  suggests  the  notochord ;  a  portion  of 


312  ZOOLOGY 

the  nervous  system  is  dorsal;  and  gill-slits  occur.  On  the  other 
hand  there  is  a  connective  around  the  esophagus  and  a  ventral 
nervous  thickened  band  as  in  Annulata,  and  it  shows  little  or 
no  signs  of  segmentation  (see  Fig.  153). 

2.  Tunicates  (sea-squirts,  ascidians,  etc.)  comprising  a 
variety  of  forms  which  may  be  said,  on  the  whole,  to  be  de- 
generate in  the  adult  condition.  It  is  in  the  larval  or  tadpole 
state  particularly  that  their  relation  to  the  Chordata  is  sug- 
gested. -In  the  larva  they  possess  a  notochord  especially  in  the 

FIG.  153. 


FIG.   153.     Balanoglossus  (Male).     After  Bateson.     a,  anus;  m,  mouth;  p,  proboscis;  po.,  pores, 
the  openings  of  the  gill-slits;  ts.,  tester. 

Questions  on  the  figure. — Make  reference  to  other  texts  and  figures  and  de- 
termine what  features  of  Balanoglossus  tend  to  ally  it  with  the  chordates.  What 
are  the  habits  of  the  animal?  Where  do  the  earlier  zoologists  class  Balanoglossus? 

tail  region,  a  dorsal  nervous  system,  and  gill-slits.  The  adult 
forms  are  usually  attached,  many  of  the  larval  organs  becoming 
much  changed  or  even  wholly  lost  in  consequence  of  the 
changed  mode  of  life.  The  adults  have  been  variously  classified 
as  worms,  mollusks,  etc.  Many  of  the  tunicates  multiply  by 
budding  and  form  colonies  from  the  fact  that  the  buds  remain 
associated. 

3.  Amphioxus  (lancelet)  possesses  the  characters  above 
mentioned  as  belonging  to  all  the  Chordata,  except  that  there 
is  no  true  heart.  In  addition  it  has  a  fish-like  body,  and  the 
muscles  are  arranged  in  segments  which  appear  externally.  It 


CHORDATA 


313 


lacks  paired  fins,  but  has  a  median  dorsal 
fin  which  continues  over  the  tail  and  for- 
ward a  part  way  on  the  ventral  side.  In 
front  of  this  is  a  thick  fold  (metapleure)  on 
either  side  the  body,  at  the  junction  of  the 
side  with  the  belly  of  the  animal.  The 
metapleure  is  thought  by  some  zoologists 
to  be  the  forerunner  of  the  vertebrate  ap- 
pendages. Amphioocus  is  without  a  definite 
brain ;  that  is  to  say,  the  anterior  end  of  the 
nervous  tube  is  not  highly  specialized.  It 
has  no  skull,  eyes,  nor  ears,  such  as  charac- 
terize the  head  of  true  vertebrates.  Amphi- 
oxus is  a  small  semi-transparent  animal 
about  two  inches  long  (Fig.  154).  It  bur- 
rows in  the  sand  with  only  the  fringed  mouth 
exposed.  It  may  vary  this  by  swimming 
about  for  short  periods. 

4.  Cyclostomes  (Lampreys). — These  are 
eel-like  animals  usually  classed  with  the 
fishes,  and  are  doubtless  more  closely  related 
to  them  than  to  the  forms  before  mentioned, 
but  because  of  their  primitive  qualities  they 
may  be  placed  for  the  purposes  of  this  course 

FIG.  154.  Diagram  of  the  anatomy  of  Amphioxus,  drawn  as  a 
semi-transparent  object  (after  Perrier  "Traiti  de  Zoologie").  a, 
anus;  a. p.,  atrial  pore;  c.f.,  caudal  fin;  dr.,  cirri,  on  the  edge  of  the 
vestibule  leading  to  the  mouth;  d.f.,  dorsal  fin;  r,  fin  rays;  g,  gill 
or  branchial  structures  consisting  of  alternate  slits,  through  which 
the  water  passes,  and  supporting  plates,  in  the  walls  of  which  are  the 
blood  vessels;  in.,  intestine,  from  which  as  a  diverticulum  springs 
/.,  the  liver;  m,  the  mouth  surrounded  by  a  fringed  velum;  my., 
myotomes  or  muscle  segments;  n.c.,  notochord;  o.,  ovaries;  s.c., 
spinal  cord;  v.f.,  ventral  fin. 

Questions  on  the  figure. — What  elements  of  struc- 
ture appear  in  the  figure  suggesting  the  chordate  charac- 
ter of  Amphioxus?  What  is  the  relation  of  the  spinal 
cord,  notochord  and  digestive  tract  ?  How  much  of  the 
length  of  Amphioxus  possesses  gills?  What  is  the  posi- 
tion, extent  and  function  of  the  atrium?  (Refer  to  more 
extended  texts.)  What  structures  show  evidences  of 
segmentation?  What  fins  has  Amphioxus?  Compare 
with  fins  of  fishes. 


FIG.  154. 


314  ZOOLOGY 

among  the  Protovertebrata.  They  have  a  round  sucking  mouth 
destitute  of  jaws ;  they  lack  paired  appendages  and  the  external 
skeleton.  There  is  only  one  nostril,  which  may  or  may  not 
communicate  with  the  pharynx.  The  cyclostomes  possess  a 
true  brain,  a  cartilaginous  internal  skeleton,  and  gills  (usually  6 
or  7  pairs)  in  pouches.  They  differ  from  the  true  fishes  in  the 
fact  that  the  notochord  is  not  constricted,  i.e.,  the  mesodermal 
sheath  does  not,  by  its  growth,  compress  it  by  the  develop- 
ment of  distinct  vertebrae  around  it  (see  §344).  See  Fig.  64. 

340.  Library  Exercises. — By  reference  to  all  the  available 
literature  make  a  report  on  the  general  structure,  habits,  and 
important  adaptations  of  each  of  the  above  types?  How  do 
the  larvae  and  adults  of  the  tunicates  compare?  How  is  the 
degeneration  accounted  for.  To  what  extent  is  colonial  life 
represented  among  these  types  ?  Are  any  parasitic  ?  Examine 
particularly  for  figures  of  these  groups  in  the  standard  refer- 
ence zoologies. 


CHAPTER  XIX 

CHORDATA   (CONT.):  SUB-PHYLUM  VERTEBRATA  (FISHES,  AMPHIBIANS, 
REPTILES,  BIRDS,  AND  MAMMALS) 

LABORATORY  EXERCISES 

For  general  illustration  of  the  vertebrates  the  author  is 
convinced  that  no  form  is  superior  to  the  frog  for  use  in  ele- 
mentary classes,  although  some  teachers  prefer  a  fish.  In 
a  course  arranged  for  one  year  it  is  not  desirable  to  make 
elaborate  dissections  of  more  than  one  or  two  vertebrate  types. 
Directions  are  given  both  for  the  fish  and  the  frog  for  the  con- 
venience of  those  teachers  who  prefer  the  former.  Supplemen- 
tary studies  for  the  other  classes  of  vertebrates  will  be  found 
in  connection  with  the  chapters  devoted  thereto. 

341.  Fish. — Any  common  fish  will  serve — perch,  sucker,  trout,  smelt.  Speci- 
mens eight  to  ten  inches  in  length  are  of  most  suitable  size.  If  convenient  one- 
half  the  class  might  take  one  species  and  the  remainder  another. 

A.  The  Living  Animal. — Place  in  a  tub  of  water,  or  better  in  a  vessel  one  side 
of  which  is  glass.     Watch  the  locomotion  and  notice  all  the  accompanying  motions 
of  the  various  parts.     What  is  the  rate  of  the  tail  stroke?     How  far,  on  an  average, 
does  one  stroke  of  the  tail  carry  the  fish?     Compare  these  points  when  the  fish  is 
in  very  rapid  motion.     What  part  do  the  anterior  fins  play  in  locomotion?     Bind 
one  of  them  flat  against  the  body  with  a  string.     Bind  both.     Results?     Experi- 
ment similarly  with  the  other  fins  and  see  if  your  first  conclusions  are  strengthened. 
Do  you  find  any  variations  in  the  above  respects  by  comparing  several  species? 

How  does  the  temperature  of  the  fish  compare  with  that  of  the  water?  Allow 
one  specimen  to  remain  for  an  hour  or  more  in  water  at  a  temperature  of  70°  F. ; 
another  in  cooler  water  (50°  F.):  compare  results. 

Can  the  fish  detect  in  the  water  the  presence  of  substances  which  have  a  decided 
taste  to  us?  Use  colorless  solutions, — acid,  sugar,  quinine.  Can  you  get  the 
animal  to  show  any  choice  as  to  food?  - 

Note  the  motions  of  mouth  and  eyes.  Can  the  fish  see  any  point  with  both 
eyes  at  once? 

B.  External  Anatomy. — (Make  careful  outline  sketches  showing  all  points  of 
structure.) 

The  Topography  of  the  Body. — Note  the  symmetry;  indicate  the  degree  of 
differentiation  of  anterior  and  posterior  ends,  and  of  dorsal  and  ventral  surfaces, 
as  shown  by  the  shape,  special  organs,  etc.  What  structures  appear  paired? 
To  what  degree  are  head,  trunk,  and  tail  clearly  distinguishable?  Locate  and 

315 


316  ZOOLOGY 

identify  all  the  external  openings.  How  would  you  describe  the  general  shape  of 
the  body?  What  are  the  external  evidences  of  segmentation? 

The  A  ppendages. — How  many  paired  ?  Unpaired  ?  Locate :  the  dorsal,  caudal, 
anal,  the  pectoral,  and  the  pelvic  or  ventral.  Does  the  skin  of  the  body  extend 
over  the  fins?  What  seems  to  be  the  nature  of  the  fin  rays?  Number?  Are  the 
upper  and  lower  lobes  of  the  caudal  fin  equal  or  unequal  ? 

The  Covering. — Does  the  specimen  possess  scales  ?  Is  there  any  regularity  in 
their  arrangement?  Is  this  constant  among  several  specimens?  Is  it  the  same 
in  different  species?  Are  any  parts  of  the  body  free  of  scales?  Are  the  scales 
covered  with  skin?  What  is  the  shape  and  nature  of  the  free  margin  of  the  scales? 
Examine  with  a  hand  lens  or  low  power  of  microscope.  Is  there  any  color?  How 
does  this  appear  under  the  microscope?  Do  you  distinguish  a  line  (lateral  line} 
along  one  of  the  rows  of  scales  on  the  side  of  the  body?  Examine  one  of  these 
scales  under  the  microscope.  How  does  it  differ  from  the  others?  How  many 
rows  of  scales  above  the  lateral  line?  Below? 

The  Head. — What  goes  to  make  up  the  head  of  the  fish?  Note  position,  shape 
and  size  of  mouth.  Find  the  bony  framework:  upper  jaw  (premaxillaries  in  front 
articulating  with  maxillaries  behind);  lower  jaw  (dentaries).  Are  both  jaws 
movable?  Locate  all  the  bones  which  bear  teeth?  How  are  teeth  arranged? 
Is  there  a  tongue?  Do  the  nostrils  communicate  with  the  mouth  cavity? 

Eyes:  number,  position,  coverings  (lids?),  iris,  pupil. 

Are  there  any  ' '  ears  ? ' '     Evidences  ? 

Gills  and  Gill-coverings. — How  many  bones  in  the  gill  cover  (operculum)? 
Describe  the  structure  at  its  inner,  lower  margin.  How  wide  is  the  isthmus 
between  the  right  and  left  gill-openings?  Identify  and  number  the  gill  arches, 
which  bear  the  reddish  gill-filaments,  and  the  gill  slits  between.  With  what  do  the 
passages  between  the  gill  arches  communicate  ?  Determine  by  extending  a  probe 
into  mouth.  How  does  the  inner  or  pharyngeal  side  of  the  gill  arches  differ  from 
that  which  bears  the  filaments?  Examine  the  gills  for  parasites: 

C.  Internal  Structure. — With  forceps  and  sharp  scalpel  remove  a  strip  of  skin 
an  inch  wide  from  one  side  of  the  fish,  from  the  belly  to  the  dorsal  fin.  Note  the 
muscular  segments  (myotomes).  Is  the  line  separating  two  of  these  a  straight  line? 
In  what  direction  do  the  muscle  fibres  run  ? 

With  scissors  cut  the  body  wall  along  the  middle  line  of  the  belly  from  just  in 
front  of  the  anus  to  the  isthmus.  Be  very  careful  not  to  injure  any  of  the  organs 
within  the  coelom.  A  portion  of  the  side  muscles  may  be  removed  on  one  side 
or  cuts  may  be  made  perpendicular  to  the  first  so  that  the  sides  may  be  more  readily 
opened.  Notice  the  lining  of  the  body  cavity  (peritoneum).  Color?  How  is 
the  heart  separated  from  the  abdominal  cavity?  (False  diaphragm.)  Sketch 
the  cavities  thus  laid  open  and  represent  the  organs  as  they  appear  before  dis- 
turbing them.  Has  the  liver  lobes? 

Examine  more  in  detail,  turning  liver  to  one  side. 

I.  Digestive  Organs. — Extend  probe  through  the  mouth  into  stomach,  and 
locate:  esophagus;  stomach,  form  and  position;  intestine,  its  point  of  origin,  course 
and  outlet.  About  the  junction  of  stomach  and  small  intestine  look  for  finger-like 
projections  from  the  surface  of  the  gut  (pyloric  c&ca).  If  present,  cut  one:  is  it 
solid  or  hollow?  Examine  the  membrane  (mesentery)  which  holds  the  intestine 
in  place.  To  what  part  of  the  body  wall  is  it  attached?  The  spleen,  a  dark-red, 
ductless  gland,  occurs  close  to  the  intestine.  Cut  intestine  an  inch  from  the  anus 


CHORD  ATA  317 

and  the  esophagus  in  front  of  stomach.     Remove,  open,  and  examine  interior. 
Figure  differences  in  different  portions.     Look  for  parasites  in  the  tract. 

2.  Reproductive  and  Excretory  Organs. — Find  the  whitish  testes  or  the  yellow 
or  pinkish  ovary  (or  ovaries).     Do  they  possess  ducts?     Where  is  the  outlet? 

Observe  position  and  dimensions  of  the  air-bladder  (if  present) ;  pierce  it  and 
discover  dorsal  to  it  the  red  kidneys.  Number,  shape,  and  dimensions  of  these? 
Can  you  find  their  outlets? 

3.  Pericardial  Cavity  and  its  Contents. 
Shape  and  boundaries  of  the  cavity. 

Heart:  position;  portions;  ventricle  (ventral)  and  auricle  (more  dorsal). 
From  the  ventricle  the  whitish  bulbus  arteriosus  passes  forward  and 
narrows  into  the  ventral  aorta.  Posterior  to  the  heart  is  the  thin-walled 
sinus  venosus  which  communicates  with  the  auricle. 

[The  teacher  may  supplement  this  work  by  a  demonstration  of  the  ventral 
aorta  with  its  branches  passing  to  the  gills,  by  means  of  a  larger  fish  in  which  these 
vessels  have  been  injected  with  a  colored  mass.  See  appendix.] 

4.  The  Nervous  System. — Cut  off  the  head  and  remove  the  muscles  from  the 
back  and  top  of  the  skull.     Use  a  strong  cartilage  knife  and  gradually  slice  and 
pick  the  bone  until  the  cavity  within  is  well  uncovered.     Note  the  loose  tissues 
covering  the  brain.     Remove  this  with  great  care. 

Beginning  in  front,  identify  as  you  pass  backward:  olfactory  lobes,  tapering 
toward  the  front  and  communicating  with  the  nasal  cavities;  cerebrum,  two  oval 
prominences  meeting  in  the  middle  line;  the  two  large,  rounded  optic  lobes;  the 
cerebellum,  a  single  median  lobe;  and  the  medulla  oblongata,  which  tapers  backward 
into  the  spinal  cord.  Is  there  any  real  boundary  between  the  spinal  cord  and  the 
medulla,  or  is  the  distinction  arbitrary?  What  is  the  size  of -the  cord  where  it 
emerges  from  the  cranium?  What  is  its  position  in  relation  to  the  vertebrae? 
Have  you  found  any  nerves  leaving  the  medulla  or  the  cord?  If  so  how  many? 
Are  there  any  cavities  in  the  brain  lobes? 

5.  The  Eye. — Remove  the  bone  from  above  the  eye  and  examine  it  in  position. 
How  is  the  eye  moved  in  life  ?     Can  you  discover  any  of  the  muscles  effecting  these 
motions?     How  are  they  attached?     What  is  the  shape  of  the  eye?     Split  it 
open,  and  find  the  lens.     Is  the  lens  more  or  less  nearly  spherical  than  you  expected  ? 

6.  The  Skeleton. — The  general  shape  and  character  of  the  skull  and  its  bones 
may  be  seen  by  boiling  the  head  of  another  fish  and  scraping  and  picking  away  the 
flesh.     The  principal  regions  are  the  cranium  or  brain  case,  the  opercular  bones  of 
the  gill  covers,  and  the  facial  bones.     Notice  the  loose  way  in  which  the  lower  jaw 
is  articulated. 

Boil  a  two  inch  block  taken  from  the  tail  of  the  fish  until  it  becomes  tender. 
Notice  incidentally  the  shape  of  the  individual  myotomes  or  muscle  segments  as 
they  fall  apart.  Clean  the  vertebrae  of  flesh,  and  study  the  structure  of  one  of 
them.  Note  the  centrum;  the  dorsal  or  neural  arch  and  spine;  the  ventral  or  hamal 
arch  and  spine.  What  is  the  shape  of  the  centrum?  What  structures  occupy 
the  arches?  Prepare  a  trunk  vertebra  and  compare  in  all  respects  with  the  caudal. 
How  are  the  ribs  related  to  the  vertebra?  Can  you  find  any  evidence  whether 
they  are  homologous  with  the  haemal  processes  ? 

Are  there  any  bones  connected  with  the  fins? 

D.  General  Questions. — What  internal  organs  show  segmentation?  Do  they 
show  it  equally  in  all  parts  of  the  body?  Do  the  internal  organs  show  bilateral 


318  ZOOLOGY 

symmetry  as  completely  as  the  external?  How  do  you  account  for  the  fact? 
Compare  the  relative  position  of  the  anterior  and  posterior  appendages  in  as  many 
species  of  fish  as  you  can  secure?  What  are  the  habits  of  the  species  you  have 
been  studying?  Feeding  habits;  spawning  and  breeding  habits?  What  are  its 
nearest  relatives  among  the  fishes? 

E.  Studies  in  Nature. — Many  most  interesting  and  instructive  problems  present 
themselves  for  the  student  of  fishes  in  their  native  haunts,  if  the  school  is  favorably 
situated.  What  species  are  found  in  the  nearby  waters?  How  do  they  differ  in 
appearance?  How  in  habits  of  feeding;  of  swimming;  of  spawning?  What  kind 
of  habitat  does  each  prefer?  What  knowledge  must  a  successful  fisherman  of  a 
given  species  have? 

342.  The  Frog.*  (Rana). — Any  species  of  frog  will  serve. 
For  internal  anatomy  as  large  specimens  as  possible  should  be 
used.  The  frog  is  especially  suitable  to  represent  the  verte- 
brates because  of  its  metamorphosis  from  a  water-breathing 
or  fish  habit  into  the  air-breathing  condition,  and  the  readiness 
with  which  the  main  facts  of  this  metamorphosis  may  be  fol- 
lowed even  by  an  elementary  class.  Frogs  may  be  kept  alive 
almost  indefinitely,  even  through  the  winter,  by  putting  them 
in  a  deep  box  covered  with  netting,  in  which  a  pan  of  water 
is  placed.  The  bottom  of  the  box  should  be  covered  by  sod 
or  moss  which  must  be  kept  moist.  Change  the  water  in  the 
pan  every  few  days.  Do  not  place  large  and  small  frogs  in 
the  same  box,  as  the  small  ones  are  more  than  likely  to  disap- 
pear. Unless  living  animals,  as  grasshoppers  and  the  like, 
can  be  given  them  it  is  scarcely  worth  while  to  try  to  feed 
them.  They  seem  to  do  quite  as  well  without  food  for  a 
reasonable  length  of  time. 

A.  The  Living  Animal  (chiefly  physiology). — One  or  more 
exercises  may  well  be  given  to  observation  of  frogs  in  their 
native  haunts,  if  the  study  comes  at  such  time  as  will  allow. 
You  will  know  enough  about  the  frog  to  set  for  discovery  a 
number  of  .interesting  things.  Keep  careful  notes  of  all  your 
questions  and  your  discoveries.  If  this  is  not  possible,  much  can 
be  gained  by  studying  living  specimens  in  the  laboratory.  Record 
what  you  know  from  observation  of  the  animal's  general  haunts 
and  habits.  To  what  extent  is  it  a  terrestrial  animal? 
Aquatic?  What  is  the  natural  position  when  at  rest?  What 
are  its  modes  of  locomotion  on  land?  Place  on  the  floor,  and 
test.  Describe  its  motions  in  water,  and  the  use  made  of  the 


CHORD ATA  319 

parts  of  the  body  in  swimming  and  in  its  other  methods  of 
locomotion.  Can  it  rest  at  the  surface  of  the  water?  How 
much  of  the  body  protrudes  from  the  water?  How  does  it 
dive?  Can  you  find  any  evidence  that  it  does  anything  to 
increase  its  specific  gravity  when  diving  ? 

Feed  by  placing  living  grasshoppers  or  flies  in  the  vessel 
with  a  frog,  or  by  dangling  a  piece  of  meat  in  front  of  it  at 
the  end  of  a  string.  Note  the  action  of  the  tongue  in  making 
the  capture.  Examine  the  mode  of  attachment  of  tongue, 
and  suggest  its  possible  advantages. 

Watch  the  animal  while  floating  at  the  surface  of  water  or 
out  of  water.  Can  you  detect  any  signs  of  breathing?  Note 
carefully  the  nostrils,  the  cheeks  and  the  sides  of  the  abdomen,, 
and  determine  how  it  gets  air  into  its  lungs.  Determine 
what  senses  are  represented  in  the  frog.  How  does  it  react 
to  salines,  acids,  sweets,  bitters?  Judging  from  the  position 
of  the  eyes  and  from  experiment,  can  a  frog  on  the  ground 
see  objects  in  all  directions?  Can  it  do  so  while  floating  on 
the  surface  of  the  water  ?  Are  the  eyes  movable  ?  Can  the 
frog  see  any  point  with  both  eyes  at  the  same  time?  What 
happens  if  a  frog  is  placed  on  his  back?  Explain.  Does  the 
frog  seem  to  have  any  sense  of  elevation'  when  he  comes  to  the 
edge  of  a  table?  How  do  you  estimate  this?  By  experiment 
and  by  reference  determine  whether  the  frog's  actions  are  at 
all  modified  by  experience. 

Select  a  small  frog  and  chloroform  it  until  quiet,  but  not 
enough  to  kill  it.  Wrap  it  in  a  wet  cloth,  and  place  on  a  support 
of  such  height  as  will  allow  the  web  to  be  stretched  over  the  open- 
ing in  the  stage  of  the  microscope.  With  the  lower  power  note 
the  pigment  cells  and  blood  vessels.  Determine  which  are 
arteries  and  which  veins;  present  your  evidences.  By  placing  a. 
little  water  and  a  cover-glass  on  the  web  the  high  power  may  be 
used,  and  the  behavior  of  the  corpuscles  studied  as  they  pass 
through  the  capillaries.  Similar  studies  may  be  made  on  the: 
gills  of  very  young  tadpoles. 

B.  External  Anatomy. — What  is  its  symmetry?  Compare 
carefully  the  structure  and  form  of  the  dorsal  with  that  of 
ventral  surface;  similarly  those  of  the  anterior  and  posterior 


320  ZOOLOGY 

ends.     Compare  several  individuals  as  to  shape,  color  markings, 
size,  etc. 

General  form. 

Head,  trunk,  limbs.     Is  there  any  neck  ? 

Anterior  appendages:  arm,  forearm,  hand  (including 
digits).  Compare  with  your  own  hand,  and  deter- 
mine which  is  the  first  digit,  or  the  thumb  side  of 
the  hand. 

Posterior  appendages:  thigh,  shank,  ankle,  foot.     How 
many  digits  ?     Which  is  the  first  ?     How  many  joints 
in  each?     What  other  peculiarities  are  noteworthy? 
Special  head  structure. 

Mouth;    position,    dimensions,    degree    of    extensibility; 

tongue ;  teeth,  where  located  ? 

Sense  organs:  position  of  eyes,  ears,  nose.     Do  the  nasal 
openings  communicate  with  the  mouth?     Pierce  the 
tympanic  membrane  in  a  dead  frog  and  discover  with 
what  the  opening  communicates. 
Cloacal  opening. 

C.  Internal  Anatomy. — Make  a  slit  in  the  skin  of  the  ventral 
surface  from  a  point  just  in  front  of  the  cloaca  forward  to  the 
throat,  a  little  to  one  side  of  the  middle  line.  Make  incisions 
perpendicular  to  this  and  turn  the  flaps  back  to  show  the 
muscles  beneath.  Is  the  skin  as  closely  attached  to  the  muscles 
as  in  the  fish  ?  Do  you  find  myotomes  as  in  the  fish  ?  Draw 
in  outline  some  of  the  more  important  muscles  of  the  chest  and 
abdomen.  Cut  the  muscular  wall  in  the  same  way,  passing  to 
one  side  of  the  breast  bone.  Turn  back  the  flaps  and  sketch 
and  identify  the  organs  in  their  position  in  the  ccelom. 

i.  Digestive  Organs. — Pressing  the  liver  aside,  identify  the 
following  parts  of  the  digestive  tube:  esophagus,  stomach, 
small  intestine,  large  intestine.  Are  there  any  sharp  boundaries 
between  these  parts  ? 

Compare  the  lengths  of  the  different  portions.  Find  the 
mesentery,  and  show  its  relation  to  the  intestines.  What  is 
the  relation  of  the  liver  to  the  digestive  tube?  Find  the  gall- 
bladder :  does  it  have  any  duct  Reading  to  the  tube  ?  What  is 


CHORD  ATA 

the  position   of   the  light   colored   pancreas?     Of   the   darker 
spleen  (this  organ  has  no  duct)  ? 

Cut  the  large  intestine  about  an  inch  from  the  anal  opening 
and  the  esophagus  in  front  of  the  stomach;  remove  the  tract 
from  the  body.  Split  it  from  end  to  end,  wash  it  of  its  contents 
and  describe  and  make  drawings  of  the  interior.  Do  you  find 
any  internal  parasites  ? 

2 .  Urino genital  Organs. — Without  removing  any  other  organs 
identify : 

The  kidneys:   color,   form,    position.     Do   they   have   any 

outlet?     The  bladder;  position  and  general  structure. 
The  fat  bodies :  position  ?     With  what  connected  ? 
In  the  male: 

Testes;    yellowish,    rounded    bodies.     With    what    organ 

connected  ? 
How  in  a  fresh  specimen  can  you  be  sure  you  have  found 

the  testes  ? 
In  the  female : 

Ovaries,  which  vary  much  in  size  and  appearance  with 
the  time  of  year.  What  are  their  position  and  at- 
tachments ? 

Oviducts.  Do  these  communicate  with  the  body  cavity  ? 
How  do  they  communicate  with  the  exterior  ?  Is  there 
any  trace  of  the  oviducts  in  the  male  ? 

3.  The  Lungs. — Open  the  mouth,  find  the  glottis,  insert  a 
blow-pipe,  and  inflate  the  lungs:  number,  position  and  shape? 
Cut  them  open  and  examine  the  inner  surface. 

4.  The  Circulatory  System. 

The  heart:  Does  this  organ  lie  free  in  the  body  cavity? 
What  is  the  shape  of  the  heart  ?  To  what  is  it  attached  ? 
Identify  the  auricles  in  front,  and  the  ventricle  behind.  Can 
you  recognize  the  aorta  arising  from  the  ventricle;  and  the 
venous  sinus  dorsal  to  the  heart  and  receiving  the  large  veins? 
How  many  chambers  to  the  heart?  Their  relation  to  each 
other  ? 

Further  study  can  be  pursued  successfully  only  by  injecting 
the  vessels  with  a  colored  mass.  If  time  allows  each  student 
may  well  inject  the  arterial  system  and  trace  its  course.  At 

21 


322  ZOOLOGY 

least  a  specimen  thus  injected  and  dissected  by  the  teacher 
should  be  used  to  demonstrate  the  three  aortic  arches  (carotid, 
systemic  and  pulmonary),  the  dorsal  aorta  and  its  chief  branches. 
5.  Muscle. — Strip  the  skin  from  the  leg  like  a  stocking. 
Without  cutting,  separate  the  muscles  from  each  other,  demon- 
strating their  general  shape  and  the  tendons  at  the  ends  by 
which  they  are  connected  with  the  bones.  The  end  attached 
to  the  least  movable  bone  is  the  origin,  the  other,  the  insertion. 
What  is  the  origin  and  what  the  insertion  of  the  large  muscles 
of  the  thigh?  Are  the  muscle  fibres  plain  or  cross-striate ? 
(Examine  a  small  bit  under  the  microscope  after  teasing  it 
apart  as  much  as  possible.)  These  studies  may  very  well  be 
extended  to  include  the  muscles  of  the  throat,  chest,  arm,  ab- 
domen, and  leg.  In  all  such  studies  the  most  important  ques- 
tion the  student  can  ask  and  answer  is  this:  "What  does  this 
muscle  do  when  it  contracts?"  To  answer  the  question  one 
must  find  the  origin  and  insertion  of  the  muscle,  as  well  as  its 
relation  to  other  muscles. 

6.  Nervous  System. — Remove  with  great  care  the  skin,  muscles  and  bone  from 
the  roof  of  the  skull  so  as  to  expose  the  brain.     Continue  backward  and  expose  the 
anterior  portion  of  the  spinal  cord.     Sketch,  as  it  appears  from  above,  and  identify, 
beginning  with  the  anterior  end: 

Olfactory  lobes. 

Cerebral  hemispheres;  number,  size,  form.     Are  they  separate? 

Optic  lobes. 

Cerebellum;  a  narrow  transverse  band. 

Medulla  oblongata,  tapering  into  the  spinal  cord. 

Examine  the  nerves  arising  from  the  spinal  cord.  Look  within  the  body  cavity 
also  for  these.  What  is  their  position  in  relation  to  the  vertebrae?  How  many 
pairs  can  you  discover?  Does  each  arise  by  a  single  or  double  root?  Find  the 
large  nerve  (sciatic)  which  is  the  chief  nerve  of  the  hind  leg.  How  many  spinal 
nerves  enter  into  the  formation  of  it  ?  Seek  a  similar  plexus  in  connection  with  the 
front  leg.  Seek  the  sympathetic  ganglia  in  body  cavity  on  either  side  of  the 
backbone. 

Dissect  the  bone  and  muscle  from  one  side  of  the  skull,  showing  the  cranial 
nerves.  Begin  at  the  anterior  end  and  identify:  (i)  the  olfactory  nerve;  cut,  and 
lift  the  brain  slightly,  showing  (2)  optic  nerves.  Cut  these  as  far  from  the  brain  as 
possible. 

Note  other  smaller  nerves  and  cut  these.  How  many  are  there?  From  what 
part  of  the  brain  do  the  majority  of  them  arise?  Do  the  optic  nerves  join  at  the 
point  where  they  enter  the  brain? 

7.  Skeleton.- — Pick  the  bulk  of  the  flesh  from  the  bones  of  an  uninjured  skeleton. 
A  few  minutes  of  boiling  will  be  of  advantage  in  the  final  stages  of  cleaning. 


CHORDATA 


323 


Identify  the  axial  skeleton  and  the  appendicular .  Do  the  appendages  unite  directly 
with  the  axial  skeleton?  Count  the  vertebrae.  To  what  extent  do  they  differ? 
Can  they  be  grouped  into  regions?  Select  a  typical  one,  and  draw  from  various 
positions  to  show  structure.  Do  any  bear  ribs?  Describe  the  posterior,  bone  in 
the  series.  Identify  the  parts  of  the  anterior  and  posterior  limbs  and  girdles  by 
referring  to  Fig.  161,  and  see  to  what  extent  they  depart  from  the  type  described 
there.  Make  outline  sketches  of  all  the  bones  of  the  right  girdles  and  appendages. 
What  is  the  nature  and  action  of  the  various  joints  of  the  limbs?  In  the  skull 
notice  how  small  a  portion  is  brain-case.  How  is  the  great  width  of  the  head 
secured?  How  is  the  lower  jaw  related  to  the  skull?  Make 'a  sketch  showing 
the  proportions  of  these  various  parts.  In  what  position  are  teeth  borne?  Ex- 
amine the  sternum  or  breast  bone.  How  related  to  the  girdle?  Of  what  parts  is  it 
composed.  How  much  of  it  is  cartilaginous? 

8.  Development. — Eggs  of  frogs  and  toads  may  be  found  in  the  early  spring  in 
ponds  or  sluggish  streams,  floating  or  attached  to  submerged  objects.  They 
occur  in  slimy  strings  or  masses,  each  egg  enveloped  in  a  jelly-like  covering. 
Transfer  these  to  the  laboratory,  and  keep  covered  with  water  in  a  shallow  vessel. 
Change  the  water  frequently,  and  keep  a  close  watch  on  the  changes  which  they 
undergo.  After  hatching  keep  a  few  water  plants  in  the  vessels  for  the  tadpoles  to 
eat. 

Note  appearance  of  the  egg  (with  low  power  of  microscope). 
Gelatine:  outer  layer,  not  really  a  part  of  the  egg. 
Fertilized  ovum;  the  darker  interior  sphere,  of  protoplasm  and  yolk. 

If  the  eggs  are  recently  laid,  the  beginning  of  segmentation  will  furnish  an 
interesting  demonstration  for  the  class.  How  are  the  first  cleavage  planes  related 
to  each  other? 

If  more  advanced,  note  especially:  the  gradual  elongation  of  the  embiyo,  the 
enlargement  of  the  head,  development  of  the  tail,  hatching,  the  external  gills. 
What  becomes  of  the  gills?  Do  you  find  any  trace  of  mouth,  eyes,  nasal  openings? 
Where  do  the  legs  first  appear?  What  becomes  of  the  tail?  Prove.  Tadpoles 
of  all  ages  may  usually  be  found  in  the  shallow  ponds  late  in  spring.  These  should 
be  compared  with  those  reared  in  the  laboratory.  Dissect  one  of  the  larger  tad- 
poles, and  examine  particularly  the  intestine  and  the  gill  chamber. 

343.  Compare  with  the  frog  any  other  amphibian  types  which  can  be  found, — 
as  the  toad,  the  newt,  or  the  salamander.  Note  especially  the  differences  in  habits, 
haunts,  external  form,  appendages,  method  and  time  of  depositing  eggs,  the  form 
of  the  tadpoles,  etc. 

DESCRIPTIVE  TEXT 

344.  General  Characters. — In  common  with  the  simpler 
Chordata  thus  far  considered  the  Vertebrata  are  bilaterally 
symmetrical  Metazoa  with  a  ccelomic  cavity,  a  notochord  de- 
rived from  the  entoderm,  gill-slits  at  some  stage  of  life,  dorsal 
nerve  tube  and  a  ventral  heart.  In  addition,  the  following 
points  may  be  given  as  distinguishing  the  true  vertebrates: 

i.  The  notochord  comes  to  be  surrounded  by  a  sheath  of 
tissue  derived  from  the  mesoderm.  This  produces  around  the 


324  ZOOLOGY 

notochord  the  internal  skeletal  axis,  the  centra  of  the  vertebrae, 
composed  either  of  cartilage  or  bone  (Figs.  156-158). 

2.  Outgrowths  from  these  centra  pass  dorsally  to  protect 
the  nerve  tube,  and  ventrally  to  protect  the  viscera  (Fig.  159). 

3.  Several  sets  of  organs  show  varying  degrees  of  meta- 
meric  segmentation:  e.g.,  vertebral  column;  muscular  system; 
nervous  system. 

4.  Jointed  appendages  having  a  central  skeleton  never  ex- 
ceed two  pairs;  one  pair  or  both  of  them  may  be  rudimentary 
or  wanting. 

5.  The  respiratory  system  is  developed  in  connection  with 
the  anterior  end  of  the  digestive  tract. 

6.  The  heart  always  has  as  many  as  two  chambers  and  the 
blood  contains  red  corpuscles. 

345.  General  Form. — While  varying  greatly  in  form,  ver- 
tebrates are  typically  elongated  animals  with  the  mouth  at  or 
near  the  anterior  extremity  of  the  long  axis.  The  position  of 
the  anus  is  variable.  It  may  be  one-half  the  length  of  the 
body  from  the  posterior  end.  The  body  is  roughly  divisible 
into  head  and  trunk  with  or  without  an  intervening  neck.  The 
neck  is  more  pronounced  in  the  land  than  in  the  water  forms. 
Posterior  to  the  trunk  containing  the  body  cavity,  there  may 
be  a  tail  into  which  the  skeleton  is  continued  but  which  is 
destitute  of  a  body  cavity. 

Bilateral  symmetry  is  shown  by  the  paired  condition  of  the 
eyes,  ears,  and  other  external  and,  to  a  less  degree,  internal 
organs.  Metamerism  on  the  contrary  is  much  more  evident 
from  the  internal  than  from  the  external  organs.  There  are 
usually  two  pairs  of  lateral  appendages  for  support  and  loco- 
motion: the  thoracic  at  the  anterior  end  of  the  trunk,  and  the 
pelvic,  ordinarily  occurring  near  the  union  of  the  trunk  and 
tail.  These  are  variously  modified  as  to  their  form  and  in- 
ternal structure  (e.g.,  fins,  legs,  arms,  wings),  but  are  looked 
upon  as  homologous.  In  many  water  forms  there  are  median 
appendages  (dorsal,  ventral,  and  caudal  fins)  also  assisting  in 
locomotion.  The  ccelom  or  body  cavity  is  well  represented  in 
the  trunk  region,  and  arises  by  a  splitting  of  the  mesoderm  into 
an  inner  layer  which  comes  to  unite  with  the  digestive  tract 


CHORD  ATA  325 

and  an  outer  layer  which  unites  with  the  ectoderm  (Fig.  156). 
In  this — the  visceral  cavity — beside  the  mesoderm-covered 
digestive  tract  to  which  reference  has  already  been  made,  lie 
the  principal  organs  of  respiration,  of  excretion,  of  circulation, 
and  of  reproduction.  Dorsal  to  the  notochord  the  nervous 
system  occupies  a  cavity  within  the  mesoderm,  which  is  not, 
however,  a  part  of  the  ccelom.  This  is  described  as  the  dorsal 
or  neural  cavity  and  is  protected  by  a  sheath  of  cartilage  or 
bone.  In  the  anterior  region  this  is  much  enlarged  to  accom- 
modate the  brain.  This  condition  of  a  dorsal  and  ventral 
cavity  is  very  characteristic  of  vertebrates.  In  mammals  the 
ventral  cavity  is  further  divided  by  the  diaphragm  into  an 
anterior  or  thoracic  and  a  posterior  or  abdominal  cavity. 

346.  Protective    and    Supportive    Structures — the    Integu- 
ment.— Covering  the  body  of  vertebrates  is  the  skin,  which 
consists  of  two  layers; — the  outer,  or  epidermis,  which  is  de- 
rived from  the  ectoderm,  and  the  dermis  or  true  skin  which  is 
mesodermal  in  origin.     The  epidermis  consists  of  from  two 
to  many  layers  of  cells  in  thickness,  and  in  the  higher  forms 
the  differentiation  into  layers  becomes  very  pronounced  (Fig. 
155,  E).     The  outermost  cells  of  the  epidermis  frequently  be- 
come hardened  for  the  better  protection  of  the  parts  within. 
This  is  especially  true  of  the  terrestrial  forms.     The  inner  layer 
of  the  epidermis  is  usually  columnar  in  form,  and  from  this 
layer  the  outer  cells  are  renewed,  and  all  special  epidermal 
growths   arise    (Fig.    155,    c.e.).     The   dermis   consists  largely 
of  connective  tissue,  but  contains  in  addition  nerves  and  blood 
vessels  beside  such  ingrowths  from  the  epidermis  as  glands, 
hair-follicles,  etc.     Fat  is  deposited  in  the  lower  layers  of  the 
denims  in  many  vertebrates. 

347.  Special  Products  of  the  Integument  often  occur  in  the 
form  of  outgrowths  or  ingrowths.     Glands  are  examples  of  the 
latter,  and  are  frequent  in  connection  with  the  epidermis.     They 
may  be  simple  and  unicellular  (mucous  glands  in  fishes)   or 
multicellular,  penetrating  deep  into  the  dermis  (sweat  and  oil 
glands,   Fig.    155,   sg).     The  mammary  glands  of  Mammalia 
are  modified  forms  of  the  oil  or  sweat  glands.      The  outgrowths 


326 


ZOOLOGY 


may  be  purely  epidermal,  as  in  hair,  feathers,  nails,  hoofs,  claws, 
and  the  scales  of  some  reptiles ;  or  in  other  instances  the  principal 
structures  are  formed  in  the  dermis,  usually  with  an  outer  layer 


FIG.  155. 


FIG.  155.  Diagram  of  the  skin  in  Mammals,  showing  the  multiple  layered  condition,  together 
with  outgrowths  and  ingrowths.  Drawn  by  Dr.  J.  W.  Folsom.  E,  epidermis;  D,  dermis;  a,  adi- 
pose tissue, — fat  deposited  amid  the  connective  tissue;  b,  blood  vessels;  c.e.,  columnar  epithelial 
layer  of  the  epidermis;/,  hair  follicle;  h,  hair;  n,  nerve;  n.e.,  nerve  ending  (sensory  corpuscle);  p, 
pore  of  sweat  gland;  seb.,  sebaceous  or  oil  gland  of  hair;  s.g.,  sweat  gland;  s.c.,  horny  layer  of  epider- 
mis; s.m.,  mucous  layer  of  epidermis. 

Questions  on  the  figure. — What  suggests  that  the  columnar  layer  of  the  epi- 
dermis is  the  most  vitally  important  layer?  Are  the  hair  and  glands  dermal  or 
epidermal  growths?  Which  structures  found  in  the  dermis  seem  to  be  invasions 
of  that  layer  by  outside  structures?  What  are  the  functions  of  the  various  layers 
of  the  skin?  Which  parts  are  ectodermal  and  which  mesodermal  in  origin? 

contributed  by  the  epidermis,  as  in  the  teeth  or  the  scales  and 
bony  plates  which  form  in  many  instances  (turtle,  armadillo, 
etc.),  a  very  complete  external  skeleton.  Some  of  the  bones 


CHORDATA 


327 


of  this  external,  or  dermal,  skeleton  persist  even  in  the  highest 
forms  (e.  g.,  man)  and  unite  with  bones  of  the  internal  skeleton, 
as  in  the  formation  of  the  cranium  and  the  facial  bones. 

The  most  apparent  function  of  the  skin  is  protection.  The 
outgrowths  (hair,  scales,  claws,  etc.),  evidently  increase  its 
adaptation  to  this  function.  In  addition,  the  skin  is  partly 
respiratory  and  excretory.  The  glands  represent  a  specializa- 
tion of  this  latter  function.  It  is  also  sensory,  and  in  an  indirect 
way  assists  in  regulating  bodily  temperature,  especially  in  the 
warm-blooded  types. 

348.  The  Skeleton. — Attention  has  already  been  called  to 
the  exoskeleton  as  the  derivative  of  the  skin.  The  endoskele- 

FIG.  156. 


coe. 


FIG.  156.  Diagram  of  transverse  section  through  embryo  of  a  Vertebrate,  showing  the  mode 
of  origin  and  the  relations  of  the  notochord,  nervous  cord,  ectoderm,  entoderm  and  mesoderm  (see 
also  Fig.  15).  coe.,  ccelomic  pouches;  eel.,  ectoderm;  ent.,  entoderm;  g,  lumen  of  the  gut;  IP.,  invagi- 
nation  of  ectoderm  which  forms  the  nerve  cord  (see  c,  in  succeeding  figures) ;  mes1.,  somatic  or  body 
mesoderm;  mes*.,  splanchnic  mesoderm,  that  portion  of  the  mesoderm  which  becomes  allied  with 
the  entoderm;  n,  the  nerve  (spinal)  cord;  n.c.,  notochord,  arising  by  an  outpocketing  of  the  entoderm. 

ton  is  surrounded  by  muscles  separating  it  from  the  integument. 
In  general  it  may  be  said  that  these  two  bony  systems  supple- 
ment each  other.  In  the  higher  forms  where  the  internal 
skeleton  is  best  developed  the  exoskeleton  is  usually  reduced 
to  a  minimum.  Elements  from  both  sources  may  become  fused 
fn  the  formation  of  a  single  structure  (the  skull;  the  carapace 
oi  the  turtle).  A  difference  between  the  internal  and  the 


328  ZOOLOGY 

external  skeleton  is  in  the  fact  that  bone  of  the  former  is  typically 
formed  in  and  around  cartilage,  whereas  in  the  latter  there  is  no 
cartilage.  The  internal  skeleton  consists  of  two  portions,  (i) 
the  axial,  embracing  the  vertebral  column,  and  (2)  the  appen- 
dicular,  or  that  supporting  the  appendages. 

As  was  seen  in  the  arthropods  the  chief  functions  of  skeletons 
are  to  protect  and  support  soft  parts,  and  to  furnish  hard  lever- 
ages for  muscular  attachment.  It  is  clear  that  any  rigid  skeleton 
which  entirely  surrounds  an  elongated  animal  must  have  some 
breaks  or  articulations  in  it.  We  found  this  to  be  true  in 
Crustacea  and  insects.  Similarly  in  the  internal  skeleton  in 
vertebrates,  the  attached  muscles  would  be  useless  if  there  were 
no  joints.  It  is  clear  that  solid  bones  coming  together  in  a  joint 
with  the  muscles  external  to  them  will  make  a  stronger  articula- 
tion than  hollow  shells  coming  together  like  the  segments  of  a 
stovepipe.  The  arthropod  joint  is  effective  for  small  animals 
but  would  not  meet  the  needs  of  the  large  vertebrates. 

349.  Axial  Skeleton. — In  its  simplest  condition  this  consists 
of  the  notochord  which  it  will  be  remembered  is  derived  from 
the  entoderm  and  lies  between  the  alimentary  canal  and  the 
spinal  cord  (Fig.  156).  In  the  true  vertebrates,  cells  arising  from 
the  mesodermal  pockets  on  either  side  (Fig.  158)  produce  a 
continuous  skeleton-forming  sheath  about  the  notochord. 
From  the  cells  of  this  sheath  are  developed,  finally,  rings  of 
cartilage  or  bone  about  the  notochord  (centrum;  pleural,  centra, 
Fig.  159,  c)  and  about  the  spinal  cord  (spinous  processes,  Fig. 
159,  no).  These,  with  certain  other  growths,  constitute  the 
typical  vertebrae.  In  this  process  the  notochord  often  becomes 
obliterated  by  the  developing  vertebrae.  To  each  vertebra 
may  be  attached  a  pair  of  ribs,  which  protect  the  ventral 
structures,  somewhat  as  the  neural  arch  protects  the  nerve  cord. 
The  ribs  of  fishes  and  of  the  higher  forms  are  not  considered 
to  be  homologous  structures  (Figs.  159,  160). 

The  axial  skeleton  varies  from  this  typical  condition  in  dif- 
ferent parts  of  its  course.  In  the  head  region,  for  example, 
the  nervous  cord  is  immensely  enlarged  and  the  neural  arches 
are  much  modified,  being  replaced  by  plates  and  supplemented 


CHOKDATA 


329 


by  the  dermal  bones.     The  following  regions  may  be  described 
as  typical : 

1.  Head  region  (skull)  embracing  the  cranium  or  brain  case 
and  its  associated  ventral  arches  including  the  bones  of  the  face. 

2.  Cervical  vertebrae,  located  in  the  neck  and  lacking  ribs. 
Usually  the  anterior  one  or  two  are  considerably  modified. 


FIG.  157. 


FIG.  158. 


-~ect. 


-mesf. 


FIG.  157.     Diagram  similar  in  position  and  lettering  to  Fig.  156,  at  a  later  stage,     c,  central  canal 

of  spinal  cord. 

Pic.  158.  Transverse  section  of  an  embryo  Vertebrate  at  a  stage  later  than  Fig.  157.  m, 
mesentery;  sk.,  the  beginning  of  the  mesodermal  skeleton  which  surrounds  the  notochord  (n.c.), 
and  in  part  the  spinal  (nerve)  cord,  n. 

Questions  on  the  figures  156  to  158. — How  does  the  mesoderm  originate  in 
vertebrates?  Trace  its  gradual  growth  and  differentiation  in  the  figures.  What 
two  principal  portions  are  to  be  distinguished?  How  does  the  notochord  arise? 
How  the  spinal  cord  ?  What  is  the  source  of  the  cavity  of  the  spinal  cord  ?  From 
which  of  the  three  layers  does  the  protecting  skeleton  arise?  What  does  the 
mesentery  connect?  What  other  organs  might  be  expected  in  the  ccelom,  if  it 
were  the  purpose  to  make  a  complete  diagram  of  the  visceral  organs? 

3.  Dorsal  vertebrae,  in  the  thoracic  region  and  bearing  well- 
developed   ribs   which   may   unite   with   a   ventral   bone,    the 
sternum. 

4.  Lumbar  vertebrae,  following  the  dorsal  vertebrae  and  not 
bearing  ribs. 

5.  Sacral  vertebrae,  usually  a  small  number  of  vertebrae,  fre- 
quently fused  into  one  piece  with  which  the  girdles  of  the 
posterior  appendages  unite. 


330 


ZOOLOGY 


6.  Caudal  vertebrae,  posterior  to  the  sacrum  and  possessing 
no  ribs. 

The  number  of  bones  in  these  regions  is  very  variable  in 
the  phylum  as  a  whole,  but,  in  the  higher  forms  particularly, 
individuals  of  related  species  present  remarkable  uniformity. 

(The  discussion  of  the  condition  of  the  skull  and  the  origin  of  its  parts  is  entirely 
too  technical  for  an  elementary  text.  The  student  should  be  referred  to  more 
advanced  works.) 

FIG.  159. 


c— 


FIG.  159.  Diagram  of  veterbrae  of  a  bony  fish.  A,  caudal;  JB,  trunk,  c,  centrum  or  body  of 
the  vertebra;  ch.,  the  notochord;  h.a.,  haemal  arch;  h.c.,  haemal  canal;  h.s.,  haemal  spine;  h.z.,  haemal 
tygapophysis,  or  articulating  facet;  m.b.,  inter-muscular  bone;  n.a.,  neural  arch;  n.c.,  neural  canal; 
».$..  neural  spine;  M.Z.,  neural  zygapophysis;  r,  rib. 

Questions  on  the  figures. — What  is  the  meaning  of  haemal?  Of  neural?  In 
life  what  occupies  the  neural  canal  ?  What  occupies  the  haemal  canal  in  the  caudal 
region?  In  the  trunk  region?  Is  there  anything  to  suggest  that  the  ribs  in  fishes 
are  homologous  with  the  processes  which  form  the  haemal  arch  (h.a.)? 

350.  The  Appendicular  Skeleton. — Here  are  embraced  the 
skeletal  parts  of  the  appendages  proper,  together  with  the  bones 
binding  them  to  the  axial  skeleton  (girdles).  Each  girdle  may 
be  said  to  consist  typically  of  three  bones,  uniting  to  form  a  joint 
with  the  first  bone  of  the  limb.  One  of  these  is  dorsal  and  the 
others  ventral  (Fig.  161,  B;  il,  is,  p.}.  The  appendages  are 
much  alike  both  as  to  their  girdles  and  the  limbs  proper.  The 
posterior  is,  in  higher  forms,  more  intimately  fused  with  the 
axial  skeleton,  thus  securing  greater  strength  at  the  expense  of 
freedom  of  motion.  The  first  joint  of  each  appendage  consists 
of  one  bone  (arm  or  thigh);  the  second,  of  two  (forearm  or 


CHORD ATA  331 

shank) ;  then  follows  a  region  of  several  small  bones  (wrist  or 
ankle),  succeeded  by  the  hand  or  foot  with  five  (usually)  bones, 
and  then  by  five  digits  (fingers,  toes)  of  a  varying  number  of 
joints.  The  accompanying  diagrams  (Fig.  161)  will  make 
clear  these  relations,  as  well  as  the  names  of  the  bones.  Bones 
may  disappear  or  fuse  with  others  in  such  a  way  as  to  cause  a 
wide  variation  from  this  type;  indeed  the  type  is  perhaps  never 
realized  in  any  single  animal.  In  fishes  the  appendage  and  the 
girdle  are  often  very  simple,  the  limb  being  little  more  than 

FIG.  160. 


FIG.  160.  Diagram  of  a  trunk  vertebra  in  a  Mammal,  c,  centrum;  ch.,  position  originally 
occupied  by  the  notochord;  h.,  head  of  the  rib;  h.c.,  haemal  cavity;  n.a.,  neural  arch;  n.c.,  neural 
canal;  n.s.,  neural  spine;  r.t  rib;  st.,  sternum;  s.c.,  sternal  cartilage  uniting  ribs  and  sternum;  t.p., 
transverse  process  of  vertebra;  tu.,  tubercle  of  rib. 

Questions  on  the  figure. — Compare  all  the  parts  here  with  corresponding  ones 
of  Fig.  159:  ^ •  B,  and  note  the  differences.  What  is  gained  by  the  articulation 
of  ribs  with  a  sternum?  What  is  lost?  In  which  groups  of  Vertebrates  is  a 
sternum  found?  In  which  are  fully  developed  ribs  found? 

radiating  fin-rays  covered  by  a  membrane  (Figs.  176,  177). 
Yet  it  is  believed  that  from  some  such  primitive  condition  the 
more  specialized  appendages  have  arisen. 

351.  The  Digestive  Organs. — As  in  many  of  the  inverte- 
brates which  we  have  studied,  the  alimentary  canal  in  the 
vertebrates  possesses  an  anterior,  ectodermal  portion  (stomo- 
dasum),  a  mid-gut  lined  with  entoderm  (mesenteron),  and  a 
posterior  ectodermal  part  (proctodaeum) .  The  tract  is  lined 
throughout  with  a  mucous  membrane.  Outside  of  this  are  the 


332 


•m.c.1  - 


t  — 


,  fa.-* 

M    ct-~n<>° 

SE^fo 


— f£. 


PIG.  161.  Diagrams  of  the  girdles  and  appendages  in  a  typical  Vertebrate.  A,  anterior;  B, 
posterior,  ac.,  acetabulum,  articulation  of  the  humerus  with  its  girdle;  c,  coracoid;  ca.,  carpals; 
c.e..  centralia;  d.c.,  distal  carpals;  d.t.t  distal  tarsals;  ^..elbow  joint;/,  fibula;/*.,  femur;/*.,  fibulare; 
gc.t  glenoid  cavity,  articulation  of  arm  with  girdle;  h,  humerus;  il.,  ilium;  in.,  intermediale;  is. 
ischium;  kn.,  knee  joint;  m.c.,  metacarpals  (1-5);  m.t.,  metatarsals  (1-5);  P,  pubis;  ph.,  phalanges 
(J-S);  Pr-c.,  precoracoid;  r,  radius;  ra.,  radiale;  sc.,  scapula;  /,  tibia;  la.,  tarsals;  <*'.,  tibiale;  «.,  ulna; 
ul.,  ulnare. 

Questions  on  the  figures. — Compare  the  two  appendages  throughout  and  note 
the  corresponding  bones.  How  much  is  girdle?  How  much  appendage  proper? 
How  many  carpals?  Tarsals?  Which  are  proximal?  Which  distal?  How  do 


CHORDATA  333 

the  phalanges  differ?  Which  is  the  thumb?  How  can  you  be  sure?  Compare 
this  figure  with  figures  (in  reference  texts)  of  the  appendages,  both  front  and  rear, 
of  the  frog;  of  some  bird;  or  some  carnivore;  of  the  horse;  of  man.  Where  are  the 
greatest  variations,  i.e.,  which  bones  depart  most  from  this  typical  condition? 

layers  of  unstriped  muscle  fibres,  circular  and  longitudinal, 
by  which  the  food  is  forced  onward.  The  muscles  are  espe- 
cially developed  in  certain  regions,  as  in  the  stomach.  Outside 
of  all  these,  in  the  portion  passing  through  the  body  cavity,  is 
the  serous  membrane  derived  from  the  mesoderm,  a  portion 
of  the  lining  of  the  body  cavity.  The  mucous  surface  which 
is,  -naturally  enough,  the  important  portion  in  digestion  and 
absorption  may  be  increased  by  the  lengthening  of  the  tube  as 
a  whole  or  by  means  of  outgrowths  (the  glands)  or  by  ingrowths 
(folds  of  various  kinds) .  The  highly  nourished  condition  of  the 
entodermal  sheet  of  cells  presumably  leads  to  their  rapid  growth 
and  foldings.  The  folds  are  often  so  arranged  across  the  axis 
of  the  tube  as  to  retard  the  progress  of  the  food  through  the 
tract,  thus  making  digestion  and  absorption  more  complete,  by 
increasing  the  time  during  which  the  food  is  exposed  to  the 
action  of  the  digestive  juices,  and  to  the  absorbing  surface. 

352.  The  Divisions  of  the  Tract. — The  mouth,  which  may  be 
either  terminal  or  ventral,  opens  into  the  buccal  cavity,  which  is 
bounded  dorsally  by  the  floor  of  the  brain  case,  on  the  sides  and 
in  front  by  the  jaws,  and  ventrally  by  a  muscular  floor  from 
which  the  tongue  arises  as  a  fold.  The  jaws  are  made  up  of 
bony  elements  from  two  sources;  a  core  of  bones  from  the  internal 
skeleton  (the  first  visceral  arch)  and  a  covering  of  dermal  bones 
which  fuse  with  it.  The  latter  are  the  bones  which  (typically) 
bear  the  teeth.  Teeth  however  occur  in  the  lower  vertebrates 
in  the  roof  of  the  mouth  or  on  the  tongue.  Their  place  may 
be  taken  by  horny  epidermal  structures,  as  in  the  beak  of  birds. 
When  present  the  salivary  glands  open  into  the  mouth  cavity. 
Posteriorly  the  buccal  cavity  communicates  with  the  pharynx, 
wliich  may  be. defined  as  the  part  of  the  digestive  tract  in  con- 
nection with  which  the  lungs  or  gills  are  developed.  In  fishes 
and  in  the  embryos  of  higher  forms  there  are  slits  in  the  side 
walls  connecting  the  pharynx  with  the  outside.  Gills  are 
developed  in  the  walls  of  these  slits.  In  forms  above  fishes  the 


334 


ZOOLOGY 


slits    become    closed    as    the    embryo    develops.     Above    the 
Amphibia  they  never  bear  gills. 

The  esophagus  is  a  narrow  muscular  tube  of  varying  length 


FIG.  162.     Stomach  of  Dog  (A)   and   of   Rat    (U).     ct   cardiac   portion;    p,   pyloric    portion;  o, 

esophagus;  *',  intestine. 

FIG.  163. 


Pic.  163.     Diagram  of  the  stomach  of  a  ruminant,     o,  esophagus;  r,  rumen  or  paunch;  re.,  reticu- 
him,  or  honeycomb;  p,  psalterium  or  many  plies;  a,  abomasum  or  rennet;  i,  intestine. 

Questions  on  figures  162  and  163. — Taken  as  a  series,  what  is  illustrated  by  the 
three  diagrams?  What  do  the  arrows  indicate?  What  is  known  of  the  function 
of  the  various  portions  of  the  ruminant  stomach? 

leading  to  the  stomach.  In  birds  an  enlarged  portion  of  it 
(the  crop)  may  serve  for  the  temporary  storing  and  softening  of 
the  food. 


CHORDATA 


335 


The  stomach  is  usually  well  differentiated  and  may  consist 
of  one  chamber  or  of  several.  In  the  latter  case  there  is  a 
division  of  labor  among  the  parts.  One  portion  may  be  highly 
muscular  and  supplied  with  a  hardened  internal  lining  for 
grinding  the  food  (gizzard  of  fowls,  Fig.  164);  in  such  in- 
stances another  portion  is  glandular.  In  the  ruminants  (ox, 
deer,  etc.),  there  are  four  chambers  in  the  stomach  (Fig.  163). 
The  gastric  glands  produce  an  acid  secretion  which  contains  a 
ferment  acting  chiefly  on  proteid  foods. 

FIG.  164. 


PIG.  164.     Diagram  of  the  stomach  and  esophagus  of  the  Fowl,     o,  esophagus;  c,  crop;  p,  pro- 
ven triculus  or  glandular  stomach;  g,  gizzard  or  grinding  stomach;  *',  intestine. 

Questions  on  the  figure. — Compare  this  figure  with  that  of  the  stomach  of 
ruminants  as  to  complexity*  What  are  the  functions  of  the  various  portions? 
What  changes  take  place  in  the  gizzard  of  flesh-eating  birds  if  they  are  forced  to 
live  on  grain?  Why  is  the  crop  located  outside  the  cavity  inclosed  by  the  ribs? 

The  food  is  retained  in  the  stomach  by  means  of  a  circular 
(sphincter)  muscle  at  its  posterior  end  where  it  narrows  into 
the  intestine.  This  latter  portion  is  the  principal  digestive  and 
absorptive  portion  of  the  tract  and  varies  much  in  length  in 
the  various  groups  in  accordance  with  the  nature  of  the  food 
used, — the  vegetable  feeders  for  the  most  part  possessing  the 
longest  intestines.  Numerous  circular  or  spiral  folds  of  the 
mucous  membrane  occur  in  the  intestine.  Special  absorptive 
organs  (mlli)  supplied  with  blood  and  lymph  vessels  may  cover 
these  folds.  Near  the  anterior  end  the  ducts  of  the  liver 
and  pancreas  open  into  the  intestine.  The  liver  is  the  largest 


336  ZOOLOGY 

of  the  glands,  and  the  pancreas  one  of  the  most  important  in 
digestion.  The  intestine  may  open  directly  on  the  exterior 
(most  mammals),  or  into  the  ectodermal  pocket  (cloaca)  which 
also  receives  the  excretory  and  genital  products  (reptiles  and 
birds). 

353.  Exercises  for  Field  and  Library. 

1.  What  difference  have  you  observed  in  the  number,  position,  and  kinds  of 
teeth  in  the  various  vertebrates  of  your  acquaintance? 

2.  Can  you  cite  from  your  observation  any  evidences  of  adaptation  of  the 
digestive  tract  to  the  peculiar  food  and  habits  of  the  animal  possessing  it  ?     Supple- 
ment by  library  references. 

3.  To  what  extent  is  food  prepared  in  the  mouth,  i.  e.t  antecedent  to  swallowing, 
in  the  various  vertebrates  whose  habits  you  have  observed? 

354.  Respiration. — As  in  all  higher  animals  there  are  two 
things  to  be  considered  in  the  respiration  of  vertebrates:  (i) 
the  exchange,  between  the  blood  and  the  external  medium, 
air  or  water,  of  carbon  dioxide  for  oxygen,  which  may  be  called 
external  respiration,  and  (2)  the  exchange  by  which  the  blood 
gives  the  cells  of  the  body  oxygen  and  receives  their  carbon 
dioxid,  or  internal  respiration.  The  former  is  usually  meant 
when  the  simple  term  respiration  is  used,  though  the  latter  is 
really  the  vital  process.  A  certain  amount  of  respiration  takes 
place  through  the  skin  in  almost  all  vertebrates.  Beside  this, 
special  devices — both  gills  and  lungs — are  developed  by  which 
the  blood  and  the  medium  containing  the  oxygen  are  brought 
into  closer  relation.  In  fishes  and  larval  amphibians  gills  are 
present;  in  most  adult  amphibians  and  in  reptiles,  birds,  and 
mammals,  only  lungs  occur. 

Gills  are  thin-walled  external  folds  or  groups  of  filaments 
bounded  by  a  mucous  membrane,  in  which  the  blood  circulates 
freely.  In  vertebrates  they  are  found  on  the  wall  of  passages 
leading  from  the  pharynx  to  the  outside  (gill-slits).  The  water 
passes  into  the  mouth  and  out  over  the  gills,  through  the  thin 
walls  of  which  the  gases  are  exchanged.  The  walls  between  the 
slits  may  be  supported  by  cartilages  or  bones  (visceral  or  gill- 
arches).  The  gill-slits  vary  from  four  to  eight  in  number.  In 
the  higher,  air-breathing  vertebrates  traces  of  the  gill-slits 
appear  in  embryonic  development,  but  they  never  bear  gills. 
(See  Figs.  32,  33.) 


CHORD  ATA  337 

355.  Lungs  arise  as  out-pocketings  of  the  ventral  wall  of 
the  pharynx.     These  may  persist  as  relatively  simple  sacs,  as 
in  the  frog,  or  by  great  growth  and  folding  they  may  become 
very  complicated,  and  thus  increase  their  surface  to  a  wonder- 
ful degree.     They  are  lined  throughout  with  the  entodermal 
epithelium.     The  blood   capillaries   are  in   contact   with   this 
layer  and  through  these  thin  walls  the  gases  are  exchanged. 
The  outer  surface  of  the  lung  is  covered  by  the  pleura,  the 
lining  of  the  body  cavity.     The  tube  connecting  the  pharynx 
with  the  body  of  the  lung  is  known  as  the  trachea.     The  upper 
or  anterior  end  of  the  trachea  is  modified  into  a  chamber 
known  as  the  larynx  in  the  air-breathing  vertebrates.     The 
epiglottis  closes  the  opening   (glottis)  from  the  pharynx  into 
the  larynx,  whenever  food  is  passing  from  the  mouth  through 
the   pharynx   into   the   gullet.     On    account    of  the  presence 
of  currents  of  air  passing  in  and  out  and  capable  of  produc- 
ing vibration,  certain  portions  of  the  tract  are  used  in  mak- 
ing    definite     sounds     whereby     the     animals     are    put  into 
communication  with  their  kind.     The  parts  so  used  are  the  lips, 
teeth  and  vocal  cords.     The  latter  are  membranous  folds  in 
the  mucous  lining  of  the  larynx,  which  may  be  brought  into 
such  a  position  as  to  close  that  organ,  in  part,  to  the  escaping 
current  of  air.     The  tense  edges  of  the  membrane  are  put  into 
vibration.     The  resulting  sound,  reinforced  or  otherwise  modi- 
fied by  the  other  organs,  is  voice.     Voice  has  considerable  evolu- 
tionary significance  in  animals ;  very  much  more  in  man. 

356.  Supplementary   Exercises   for   Library. — Where   does 
the  "swim-bladder"  in  fishes  occur?     Is   anything  known  of 
its  function?     Is    anything    like    a    lung    known   among   the 
fishes  ? 

What  are  the  most  important  differences  between  the  ' '  voice- 
box"  of  mammals  and  that  of  birds?  Have  all  the  vertebrate 
groups  vocal  organs?  Do  they  all  have  voice?  What  is  the 
difference  between  voice  and  speech?  What  are  the  uses  of 
voice  to  animals  possessing  it  ? 

357.  Circulation. — The  blood  in  vertebrates   contains  both 
colorless    and    colored    (red)    corpuscles.     The    red    coloring 
matter  (hemoglobin)  has  an  affinity  for  oxygen  and  thus  be- 


338 


ZOOLOGY 


comes  a  vehicle  for  transporting  it.  The  colored  corpuscles 
have  no  motion  of  their  own,  but  are  carried  by  the  blood  cur- 
rents. The  colorless  cells  are  much  less  numerous  than  the  red 
and  have  power  of  independent  motion  (amoeboid).  The  fluid 
in  which  the  cells  float  is  called  the  plasma  and  carries  the 
food  and  waste  materials  of  the  body  in  solution. 

The  muscular  heart  always  consists  of  at  least  two  cham- 
bers,  (i)  an  auricle  which  receives  blood  from  the  veins,  and 


S.  V. 


au. 


FIG.  165.  Diagrams  of  the  structure  of  the  heart  in  the  lower  Vertebrates.  At  primitive  con  - 
dition;  B,  the  position  of  the  parts  in  the  fishes,  a,  artery;  au.,  auricle;  c,  conus  arteriosus  with 
valves;  s.v.,  sinus  venosus;  t»,  valves;  ve.,  vein;  vent.,  ventricle.  The  dorsal  portion  of  the  heart  is 
toward  the  bottom  of  the  figure. 

Questions  on  the  figure. — Which  side  of  the  figure  represents  the  anterior? 
Compare  the  walls  of  the  vessels.  Where  are  the  valves  located?  How  is  the 
term  "sigmoid  flexure"  appropriate  to  the  form  in  B?  Notice  how  it  results  in 
what  is  morphologically  the  posterior  portion  of  the  heart  becoming  anterior. 
Trace  the  course  of  the  flow  of  the  blood. 

(2)  a  ventricle  which  has  thick  walls  and  propels  the  blood 
into  the  arteries.  Morphologically  the  auricle  is  the  posterior 
portion  of  the  heart  (Fig.  165,  A),  but  in  development  the  heart 
has  undergone  an  s-shaped  bending  which  has  brought  the 
auricle  in  front  of  the  ventricle  (Fig.  165,  B).  The  veins  and 
arteries  are  often  specially  enlarged  and  modified  in  the  region 
of  the  heart.  The  main  trunk  leaving  the  heart  is  called  the 
aorta.  As  the  vessels  are  followed  from  the  heart  they  branch 
and  become  smaller  and  the  walls  become  thinner.  The  final 


CHORDATA 


339 


divisions  are  the  capillaries  through  the  thin  walls  of  which 
the  blood  exchanges  materials  with  the  tissues  (Fig.  34,  c.s.; 
c.r.).  The  capillaries  unite  to  form  the  smaller  veins  and  these 


FIG.  1 66. 


FIG.  167. 


c.  v.  r._ 


c.  v.  I. 


FIG.  166.  Diagram  of  the  heart,  the  branchial  arches,  and  the  principal  veins  in  the  Teleosts. 
Ventral  view.  The  heart  is  represented  without  the  sigmoid  flexure;  that  is,  with  the  auricle 
posterior.  The  same  is  true  of  Figs.  167  to  170.  a,  aorta;  au.,  auricle;  6r.o.,  branchial  arches  of  the 
aorta  (1-4,  numbering  from  the  front);  c,  carotid;  c.v.,  cardinal  veins  (right  and  left);  d.a.,  dorsal 
arteries;  j,  jugular  veins;  d.c.,  ductus  Cuvieri;  s.v.,  sinus  venosus;  v,  ventricle.  Only  four  arterial 
arches  are  shown. 

Questions  on  the  figure. — Refer  to  the  table  on  page  343  and  identify  the  parts 
there  described.  Compare  this  figure  with  those  following  (Figs.  167-171). 
Compare  also  with  Figs.  181  and  182,  Ch.  XX.  Which  is  the  anterior  and  which 
the  posterior  portion  of  this  and  the  following  figures? 

FIG.  167.  Diagram  of  heart  and  branchial  arches  in  Ceratodus  (one  of  the  Dipnoi).  Position 
and  lettering  as  in  Fig.  166.  a.b.,  air  bladder  (lung);  p.a.,  pulmonary  artery;  p.c.,  post  caval  vein 
(right);  p.v.,  pulmonary  vein. 

Questions  on  the  figure. — What  organs  appear  in  this  diagram  which  are  not 
present  in  Fig.  166?  What  changes  of  the  various  portions  do  you  note  in  com- 
paring the  two  figures? 

uniting,  complete  the  circuit  back  to  the  heart.  It  is  evident 
that  the  capillaries  are  the  most  important  portion  of  the 
system,  the  part  for  which  the  rest  in  reality  exists. 


340 


ZOOLOGY 


In  all  the  vertebrates  there  are  certain  structures  called 
ductless  glands.  These  are  always  well  supplied  with  blood 
vessels.  They  seem  to  manufacture  and  to  pour  into  the  blood 


FIG.  168. 


FIG.  169. 


C  -- 


pre.  c 


post 


ab. 


FIG  1 68.  Diagram  of  the  heart  and  branchial  arches  in  Protopterus  (one  of  the  Dipno). 
Position  and  lettering  as  in  the  preceding,  pre.c.,  precaval  vein,  made  up  of  right  and  left  jugulairs, 
subclavians,  etc.;  post.c.,  postcaval,  made  up  of  the  cardinals,  right  and  left. 

Questions  on  the  figure. — What  are  the  chief  differences  between  the  con- 
ditions here  and  in  the  preceding  figures:  (i)  as  to  the  heart:  (2)  as  to  arteries;  (3) 
as  to  veins;  (4)  as  to  lungs? 


FIG.    169. 


Diagram    of    the  heart  and  branchial  arches  in  the   Frog. 
/.,  lungs;  l.a.,  left  article;  r.a.,  right  auricle. 


e.g.,  carotid  gland; 


Questions  on  the  figure. — How  does  the  heart  differ  from  that  of  the  Dipnoi? 
How  many  branchial  arches  of  the  aorta  are  shown?  What  evidences  can  you 
find  by  comparison  that  the  pulmonary  arch  is  derived  from  the  3d  or  4th  branchial  ? 
What  evidences  that  the  carotid  and  systemic  are  the  first  and  second  respectively  ? 
Compare  with  the  table  on  page  343.  Is  there  anything  to  indicate  that  the  im- 
purest  blood  in  the  heart  will  go  to  the  lungs? 

directly  certain  secretions  that  have  effect  on  even  remote  parts 
of  the  body.  These  glands  may  properly  be  discussed  as  a 
part  of  the  circulation  since  they  have  no  other  outlets.  These 


CHORDATA 


341 


secretions  are  called  hormones  or  internal  secretions.  The 
spleen,  the  thyroid,  the  thymus,  adrenal  bodies,  and  pituitary  body, 
are  the  chief  ductless  glands. 


FIG.  170. 


FIG.  171. 


— c. 


frrec.- 


FIG.  170.     Diagram  of  the  heart  and  branchial  arches  in  a  Reptile.     Position  and  lettering  as  in 
preceding  figures,     l.v.,  left  ventricle;  r.v.,  right  ventricle. 

Questions  on  the  figure. — Compare  this  with  figures  166-169  and  make  a  note 
of  the  differences.  How  much  communication  is  there  between  the  two  sides  of 
the  heart?  What  tends  to  insure  that  the  purest  blood  in  the  heart  shall  go  the  to 
head?  That  the  least  pure  goes  to  the  lungs? 

FIG.  171.  Diagram  of  the  heart  and  the  branchial  arches  in  Mammals.  A  dotted  outline  of 
the  arches  of  the  Fish  is  drawn  for  ready  comparison.  The  auricles  are  represented  in  a  posterior 
position,  as  in  the  preceding  figures. 

Questions  on  the  figure. — What  changes  in  the  heart  are  shown  in  this  as  conv 
pared  with  former  figures?  In  the  systemic  branchial  arch?  Remember  that  the 
heart  is  not  represented  in  its  normal  position;  the  auricles  are  really  at  the  anterior 
of  the  heart  (see  Fig.  166).  Compare  this  condition  with  table,  page  343.  What 
are  the  grounds  for  believing  that  the  auricles  are,  morphologically,  the  posterior 
part  of  the  heart? 

The  spleen  is  a  dark  body  on  the  mesentery  near  the  begin- 
ning of  the  small  intestine.     In  its  cavities  old  red  blood  cor- 


342  ZOOLOGY 

puscles  are  broken  down,  and  probably  white  blood  cells  are 
produced.  The  thyroid  and  thymus  glands  are  paired  glands 
which  are  developed  in  the  neck  in  relation  to  the  embryonic 
branchial  arches.  The  function  of  the  thymus  is  uncertain. 
There  is  iodine  in  the  thyroid  secretion.  In  man  the  disease 
or  removal  of  the  thyroid  produces  serious  disturbances  in 
development.  It  may  produce  physical  degeneracy  and  defor- 
mity, and  idiocy. 

The  adrenal  bodies  are  placed  near  the  kidneys,  as  the  name 
indicates.  They  secrete  into  the  blood  a  substance  known  as 
adrenalin  which  among  other  things  causes  the  muscles  around 
the  blood  vessels  to  continue  in  a  state  of  contraction.  In 
this  way  the  blood  pressure  is  controlled.  Removal  produces 
derangement  of  functions  and  death. 

The  pituitary  body  is  in  part  a  ventral  outgrowth  from  the 
floor  of  the  brain  (see  inf.,  Fig.  172,  B).  This  fuses  with  tissues 
arising  from  the  pharynx.  Excess  or  deficiency  of  the  secretion 
of  this  organ  causes  changes  in  the  growth  rate  of  certain  organs 
and  in  a  number  of  the  metabolic  processes.  Removal  of  the 
organ  in  dogs  causes  functional  disturbance  and  death  within 
a  fortnight. 

358.  In  Figs.    1 66  to   171   will  be  found  diagrams  of  the 
circulation  in  the  principal  groups  of  vertebrates.     It  will  be 
seen  that  there  is  a  progressive  complication  of  the  structure, 
involving  the  heart,  veins,  and  arteries,  as  we  ascend  the  scale. 
These  changes  accompany  and  are  partly  caused  by  the  change 
from  gills  to  lungs.     See  also  the  accompanying  table.     Locate 
the  vessels  and  trace  the  changes  indicated  by  means  of  larger 
texts. 

359.  In  fishes  the  blood  passes  through  the  heart  only  once 
in  making  the  circuit  of  the  body.     In  all  the  air-breathing 
forms  at  least  a  part  of  it  returns  twice,  passing  from  the 
heart  to  the  lung,  then  back  to  the  heart,  and  thence  to  the 
system  and  back  to  the  heart  again.     In  amphibia  and  reptiles 
the  blood  from  the  lung  and  from  the  system  mix  somewhat 
in  the  heart,  because  the  partition  between  the  right  and  left 
sides  is  not  complete,  but  in  birds  and  mammals  the  two  sides 


CHORDATA 


343 


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Fourth  visceral  arch; 
second  branchial. 

Fifth  visceral  arch; 
third  branchial. 
k  Sixth  visceral  arch; 
fourth  branchial. 

Superior  cardinals  1  Ductus 
[right  and  left]  1  Cuvieri 
Inferior  cardinals  |  [right 
[right  and  left],  j  andleft]. 

Hepatic  portal;  veins  from 
stomach  and  intestine  to 
liver. 

Renal  portal;  veins  from  the 
capillaries  of  the  posterior 
extremities  breaking  up  in 
the  capillaries  of  the  kid- 
neys. 

«l            }jj 

344  ZOOLOGY 

of  the  heart  are  completely  separated  and  the  pure  and  impure 
blood  are  not  allowed  to  mix  (see  Figs.  169,  170). 

360.  Excretion. — We  have   seen    that    carbon    dioxid,  one 
of  the  waste  products  of  the  protoplasmic  activity,  is  eliminated 
through  the  lungs  and  skin.     Water  is  similarly  excreted.     The 
most  important  remaining  waste  (e.g.,  urea)  contains  nitrogen. 
This  is  taken  from  the  blood  by  means  of  the  kidneys,  a  pair 
of  organs  very  complicated  both  as  to  structure  and  develop- 
ment.    They  lie  near  the  middle  line  of  the  body  at  the  back 
of  the  body  cavity.     Each  gland  represents  a  large  number 
of  nephridia  or  tubules  similar  in  some  respects  to  the  seg- 
mental  organs  of  worms   (Fig.   35),   but  much  more  compli- 
cated.    The   kidneys    are   always   well    supplied    with    blood. 
In  fishes,  amphibians,  reptiles,  and  birds  both  arteries  and  veins 
carry  blood  to  the  kidney;  in  mammals,  only  arteries.     The 
excretion,  more  or  less  in  solution  in  water,  is  poured  by  the 
tubules  into  a  duct — the  ureter — which  may  be  the  final  outlet; 
or  the  ureters  may  empty  first  into  a  urinary  bladder,  which 
has  its  own  outlet  (the  urethra). 

361.  Reproduction. — With   a   very   few   exceptions   among 
the  fishes  the  sexes  are  separate  in  all  the  vertebrates.     The 
sexual   elements   are   derived  from  modified  portions  of  the 
lining  of  the  body  cavity  (germinative  epithelium).     This  layer, 
supported  by   connective  tissue,   forms  the  essential  part  of 
the  ovaries  and  testes,  of  which  there  is  usually  a  single  pair. 
The  eggs  vary  in  size  from  Jf  oo  of  an  inch  in  mammals  to  5 
inches  (ostrich),  or  more  in  some  extinct  birds.     The  outlets 
for  the  ova  and  spermatozoa  (oviducts  and  vasa  dejerentia)  are 
modified  portions  of  the  embryonic  excretory  and  kidney  ducts. 
Throughout  the  group  there  is  a  close  connection  between  the 
excretory  and  the  reproductive  organs.     The  oviducts  may  have 
special  glands  for  depositing  nutritive  or  protective  material 
about  the  egg  before  or  after  fertilization  (as  the  albumen  and 
shell  in  egg  of  birds).     Fertilization  is  external  in  most  fishes 
and  some  amphibia,  and  internal  in  the  higher  groups.     Natur- 
ally it  must  be  internal  in  all  forms  in  which  large  masses  of  food 
or  shells  come  to  surround  the  ovum  proper,  since  the  sperm 


CHORD  ATA  345 

could  not  penetrate  these  after  they  are  once  formed.  Similarly 
internal  fertilization  would  be  necessary  in  all  cases  where  the 
young  are  developed  within  the  body  of  the  mother.  In  all 
such  there  are  special  organs  and  special  instincts  that  lead 
to  the  introduction  of  the  sperm  into  the  body  of  the  female 
(copulation).  This  however  must  not  be  confused  with  fer- 
tilization. The  uterus  is  a  special  portion  of  the  oviduct  where 
early  embryonic  development  may  occur.  (See  Figs.  205,  206.) 

362.  Development. — Those  eggs  which  are  fertilized  out- 
side develop  principally  by  means  of  the  yolk  of  the  ovum. 
Those  internally  fertilized   may   receive,    after  impregnation, 
additional  materials  for  the  further  nourishment  of  the  embryo, 
as  above  noticed  for  reptiles  and  birds.     The  fertilized  ova 
may  be  retained  for  a  longer  or  shorter  time  in  the  oviduct 
or  in  some  modified  portion  of  it  (uterus,  in  mammals)  and 
undergo    development    there.     Where    this    internal    develop- 
ment is  slight  (as  in  birds)  the  animals  are  described  as  oviparous; 
where  it  is  considerable,  as  in  mammals,  and  the  young  are 
free  at  birth  they  are  described  as  viviparous. 

The  table  on  page  346  will  give  some  of  the  facts  concerning  the  early  develop- 
ment of  vertebrates.  It  will  be  found  an  excellent  exercise  for  the  students  to 
verify  the  data  collected  in  this  and  the  preceding  table  (p.  343).  It  can  readily 
be  supplemented  by  a  demonstration  of  figures  from  more  advanced  texts. 

363.  The  Muscular  System. — We  have  seen  above  (§345) 
that  the  internal  layer  of  the  mesodermic  pockets  comes  to 
be  united  with  the  digestive  tract  and  furnishes- the  non-striped 
muscle  fibres  of  its  walls.     The  external  portion,   which  be- 
comes associated  with  the  ectoderm,  gives  rise  to  the  muscles 
of  the  body- wall  and  those  which  move  the  skeleton.     The 
fibres  of  these  muscles   are   cross-striped   or  voluntary    (Fig. 
30).     It  is  by  means  of  them  that  locomotion  is  effected.     The 
skeletal  muscles  may  be  classed  as  (i)  axial,  and  (2)  appen- 
dicular.     The  axial  are  well  shown  in  Amphioxus  (Fig.  154)  and 
the  fishes,  where  the  whole  body  is  made  up  of  repeated  seg- 
ments   (myotomes)    of   muscle   fibres.     The   muscles   segments 
alternate  with  the  segments  of  the  spinal  column,  as  one  would 
expect.     The  appendicular  muscles  are  those  which  move  the 


346 


ZOOLOGY 


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CHORD ATA  347 

limbs.  Their  general  arrangement  will  be  seen  from  the  study 
of  the  frog.  In  the  higher  vertebrates  the  segmentation  of  the 
axial  muscles  becomes  less  conspicuous  especially  in  the  head 
region,  and  the  appendicular  muscles  become  relatively  of 
greater  importance  because  of  the  greater  use  of  the  appendages. 
The  muscles  associated  with  the  entoderm  are  unstriate  and  not 
under  conscious  control. 

364.  The  Nervous  System. — The  nervous  system  in  verte- 
brates  consists   of   two   portions,    the   central  and   peripheral. 
The  central  nervous  part  embraces  the  deep-seated  organs,  the 
brain  and  spinal  cord,  and  has  for  its  most  characteristic  feature 
numerous  ganglion  cells.     From  these  central  cells  the  cell-proc- 
esses or  fibres  pass  to  the  various  tissues  of  the  body,  terminat- 
ing in  a  manner  appropriate  to  the  special  case  whether  it  be  a 
muscle,  sense  organ,  or  gland.     These  nerves  and  their  endings 
constitute  the  peripheral  part  of  the  system. 

365.  The  Central  Organs. — The  central  nervous  system  origi- 
nates from  the  ectoderm  as  a  longitudinal  groove-like  depres- 
sion in  the  mid-dorsal  line  of  the  embryo.     The  union  of  the 
edges  of  this  fold  produces  a  tube  and  an  overgrowth  of  the 
ectoderm  separates  it  from  the  outside  world  (Fig.   156).     It 
becomes  surrounded  by  mesodermal  elements  (bone  and  con- 
nective tissue,   Fig.  158),  and  itself  undergoes  numerous  and 
complex  changes.-    At  the  anterior  end  of  the  tube  occur  three 
distinct  enlargements  (Fig.  172,  A).     These  are  known  as  the 
primary  vesicles  of  the  brain,   and  by  the  later  growth  and 
differentiation  of  their  walls  they  give  rise  to  the  five  brain- 
regions  of  the  adult.     The  brain  must  be  considered  merely  as 
the  specially  modified  anterior  portion  of  the  spinal  cord. 

Three  sets  of  changes  occur  in  the  development  of  the  adult 
vertebrate  brain  from  this  primitive  condition : 

1 .  The  axis  becomes  more  or  less  curved,  the  concavity  being 
ventral. 

2.  Out-pocketings   of   the  walls   occur,    in   special  regions, 
whose  cavities  (ventricles)  retain  connection  with  the  central 
cavity  (e.  g.,  the  hemispheres).     See  Fig.  172,  h,  pi. 


348 


ol. 


PIG.  172.  Diagrams  of  the  brain  in  Vertebrates,  bowing  the  primitive  regions  a  d  their  chief 
modifications.  A,  early  stages,  showing  the  anterior  enlargement  in  three  lobes:  I,  II,  andJIIt  the 
primary  vesicles.  B,  a  sagittal  section,  showing  the  more  fundamental  modifications  of  the  walls 
of  the  primary  vesicles.  C,  frontal  section  of  same.  D,  lateral  view  of  the  brain  of  the  Frog.  The 
vertical  lines  indicate  corresponding  points  in  the  different  diagrams,  i,  2,  3,  and  4  are  the  ven- 
tricles of  the  brain,  c.c.,  crura  cerebri;  cer.,  cerebellum  (hind  brain);  h,  hemispheres  (cerebrum  or 
fore  brain);  inf.,  infundibulum;  med.,  medulla  oblongata;  mes.,  mesencephalon  or  mid  brain;  met., 
metencephalon  or  hind  brain;  my.,  myencephalon  or  medulla;  n1,  olfactory  nerve;  n*,  optic  nerve; 
ol.,  olfactory  lobe;  op.,  optic  lobes;  o.th.,  optic  thalamus;  pr.,  prosencephalon,  or  fore  brain;  £.»., 
pons  Varolii;  s.c.,  spinal  cord;  th.,  thalamencephalon  or  "between"  brain. 


CHORDATA 


349 


Questions  on  the  figures. — What  portions  of  the  adult  brain  are  produced 
from  each  of  the  three  primary  lobes?  Where  are  the  principal  outgrowths, 
thickenings  and  thin  portions  of  the  wall?  In  comparison  with  figure  D  what 
portions  of  the  brain  become  highly  developed  in  the  higher  Vertebrates?  Make 
a  diagram  based  on  D,  which  will  show  the  general  relation  of  these  parts  in  man. 
Compare  the  diagrams  with  the  table  on  page  350,  and  verify  the  statements  there. 

3.  Thickenings  or  thinnings  of  the  roof,  sides,  or  floor  of 
the  tube  may  produce  lobes  and  affect  the  size  of  the  cavity  of 
the  tube.  The  accompanying  diagrams  and  table  will  furnish 
an  outline  from  which  the  teacher  may,  if  he  desire,  pursue 
the  details  somewhat  further. 

FIG.  173. 


mcd. 


FIG.  173,  Diagram  of  head  and  brain  of  human  foetus  six  weeks  old  (heavy  boundaries).  The 
dotted  line  indicates  the  outline  of  the  brain  of  a  foetus  three  months  old.  Note  the  great  growth 
of  the  hemisphere  (A),  cer,  cerebellum;  med,  medulla  oblongata;  mes,  mesencephalon;  p,  pituitary 
body;  pr,  prosencephalon;  s.c.,  spinal  cord;  th,  thalamencephalon;  i,  olfactory  nerve;  a,  optic  nerve. 
Compare  with  Fig.  172. 

Questions  on  the  figure. — Locate  point  by  point  the  corresponding  regions  in 
the  diagrams  in  Fig.  172.  What  are  the  chief  points  of  modification?  Note  par- 
ticularly the  great  flexures  of  the  organ.  Where  is  the  cerebellum  of  the  older 
embryo?  What  is  the  nature  of  the  pituitary  body  in  the  adult? 

These  diagrams  do  not  give  the  exact  proportions  between 
the  various  parts  of  the  vertebrate  brain.  The  student  is 
urged  to  examine  figures  less  diagrammatic  in  their  nature  in 
the  larger  texts.  See,  Fig.  173;  also,  Edinger:  "Anatomy  of 


350 


ZOOLOGY 


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351 


the  Central  Nervous  System  in  Man  and  Vertebrates,"  Figs.  21, 
107-112. 

366.  That  portion  of  the  central  nervous  system  not  enclosed 
in  the  skull  is  called  the  spinal  cord.  It  is  surrounded  and  pro- 
tected by  the  dorsal  arches  of  the  vertebrae.  The  cord  is  nearly 
circular  in  cross  section,  is  somewhat  enlarged  in  the  regions  of 
the  appendages,  tapers  toward  the  posterior  end  and  is  divided 
into  symmetrical  right  and  left  lobes  by  a  dorsal  and  a  ventral 
longitudinal  groove  (see  Fig.  174,  df.).  It  possesses  a  central 


FIG.  174.  Diagram  of  a  cross-section  of  the  spinal  cord  through  the  roots  of  spinal  nerves. 
Drawn  by  Folsom.  c,  central  canal;  d.f.,  dorsal  fissure;  d.r.,  dorsal  root  of  spinal  nerve  arising  from 
the  dorsal  horn  of  the  gray  matter  (g);  gn.,  ganglion  on  the  dorsal  root;  n,  spinal  nerve;  r./.,  ventral 
fissure;  p.r.,  ventral  root  of  the  spinal  nerve,  arising  from  the  ventral  horn  of  the  gray  matter;  w., 
white  matter.  (The  dorsal  fissure  in  the  diagram  is  broader  than  it  should  be.) 

Questions  on  the  figure. — What  is  the  structural  difference  between  the  white 
and  gray  matter  in  the  cord?  Describe  their  arrangement.  How  are  the  two 
halves  of  the  cord  united?  Which  are  sensory  and  which  motor  roots?  What 
structural  differences  do  you  notice  in  the  roots? 

canal  continuous  with  the  cavities  of  the  brain.  The  outer 
part  of  the  cord  (Fig.  174,  w.)  is  composed  of  the  white  matter 
(longitudinal  nerve  fibres)  and  the  interior  portion,  of  gray 
matter  (a  mixture  of  nerve-cells  and  fibres) .  This  is  somewhat 
the  reverse  of  the  condition  found  in  the  brain. 

367.  Peripheral  Nervous  System — Spinal  Nerves. — Groups 
of  nerve  fibres  spring  from  the  gray  matter  of  the  cord  and  pass 
to  the  organs  of  the  body.  These  nerves  arise  in  pairs — one 
pair  to  each  body  segment — and  pass  out  between  the  vertebrae. 
Each  nerve  has  two  "roots,"  a  dorsal  and  a  ventral,  from  each 
of  which  some  of  its  fibres  come  (Fig.  174,  d.r.,  v.r.).  The  roots 


352  ZOOLOGY 

differ  in  appearance  in  that  the  dorsal  has  an  enlargement 
(ganglion)  containing  nerve  cells;  the  ventral  has  none.  The 
fibres  from  these  two  roots  combine  to  form  the  nerve,  but  each 
fibre  remains  independent  throughout.  It  is  known  by  experi- 
ment that  the  fibres  of  the  dorsal  root  carry  impulses  toward  the 
spinal  cord  ("sensory")  and  those  of  the  ventral  root  carry 
impulses  from  the  cord  ("motor").  In  certain  regions  the 
nerves  springing  from  successive  segments  of  the  body  may  have 
nurnerous  interlacing  fibres,  forming  what  is  known  as  a  plexus. 

368.  Cranial  Nerves. — Those  nerves  arising  from  the  brain, 
that  is,  inside  the  cranium,  are  called  cranial  nerves.  There 
are  ten  to  twelve  pairs  of  these,  but  they  are  not  of  equal  mor- 
phological value,  nor  are  they  strictly  equivalent  to  the  spinal 
nerves.  Some  have  dorsal  and  ventral  roots,  but  a  much  larger 
number  have  only  one  group  of  roots,  either  dorsal  or  ventral. 
Some  are  purely  sensory  nerves,  some  are  motor,  and  some  are 
mixed.  How  these  nerves  are  related  to  the  segments  of  which 
the  head  is  believed  to  be  composed  is  yet  an  unsettled  question. 

The  first  or  olfactory  pair  arises  from  the  olfactory  lobe  of  the  fore-brain ;  its 
fibres,  which  are  purely  sensory,  are  distributed  to  the  lining  of  the  nose,  the  end 
organ  of  smell. 

The  second  or  optic  nerve  arises  from  the  second  division  of  the  brain  (thalam- 
encephalon),  consists  of  purely  sensory  fibres,  and  is  distributed  to  the  retina 
of  the  eye,  the  end  organ  of  vision. 

The  third,  fourth,  and  sixth  pairs  are  purely  motor  and  are  distributed  to  the 
muscles  of  the  eye.  The  third  and  fourth  arise  from  the  third  division  of  the  brain 
(mesencephalori).  The  sixth  nerve  arises  from  the  medulla,  as  do  the  following. 

The  fifth  (trigeminal)  comes  from  the  anterior  portion  of  the  medulla  (hind- 
brain)  and,  like  the  spinal  nerves,  has  both  dorsal  and  ventral  roots.  It  is  largely 
sensory,  supplying  the  skin  of  the  face,  mouth  and  tongue.  Motor  fibres  pass  to 
the  muscles  of  the  jaw. 

The  seventh  (facial)  is  largely  motor,  is  distributed  chiefly  to  the  muscles  of  the 
face  and  controls  facial  expression. 

The  eighth  or  auditory  is  sensory,  and  is  distributed  to  the  inner  ear,  the  end 
organ  of  hearing  and  of  equilibration. 

The  ninth  or  glossopharyngeal  is  a  mixed  nerve  and  is  distributed  to  the  muscles 
and  mucous  membrane  of  the  pharynx  and  to  the  tongue. 

The  tenth  or  vagus  arises  by  numerous  roots,  has  both  motor  and  sensory  fibres, 
and  is  the  most  widely  distributed  nerve  in  the  body.  Its  fibres  pass  to  the  pos- 
terior visceral  arches,  lungs,  heart,  stomach  and  intestines. 

We  find  the  cranial  nerves  and  their  nerve  endings  concerned  chiefly  with  the 
higher  senses,  the  muscles  of  expression,  and  the  sensations  and  activities  involved 
in  the  fundamental  processes  of  nutrition. 


CHORD  ATA  353 

369.  The  Sympathetic  System  which  is  always  distributed  to  the  viscera 
organs  is  made  up  of  a  series  of  connected  ganglia  in  the  dorsal  part  of  the  body 
cavity.  This  system  is  in  connection  at  various  places  with  the  central  nervous 
system.  It  serves  to  connect  the  internal  organs  more  intimately,  and  is  reflex  in 
its  action. 

370.  The    Special    Senses. — The    sense    organs    represent 
specialized  terminations  of  the  nerve  fibres,  or  special  epithelial 
cells  which  have  become  associated  with  such  fibres  (Fig.  43). 
Prom  the  very  nature  of  the  case  they  must  be  external.     In  the 
case  of  higher  animals,  the  more  complicated  sense-organs  are 
removed  from  the  surface  and  are  much  modified,   but  the 
essential  sensory  portion  is  similar  in  all,  and  they  retain  some 
suitable  connection  with  the  outside.     It  is  usually  these  ac- 
cessory structures  which  transmit  the  stimuli  to  the  nerves  that 
render  the  sense  organ  so  complicated. 

371.  The  Skin  Senses. — Scattered  over  the  body  of  many 
forms  of  animals  are  single  cells,  or  groups  of  cells,  or  free  nerve 
endings,  which  are  for  the  reception  of  contact  and  temperature 
stimuli.     These  are  not  equally  numerous  or  well  developed  in 
all  parts  of  the  body.     They  are  often  especially  developed  in 
connection  with  hairs.     In  the  lower  aquatic  vertebrates,  espe- 
cially the  fishes,  groups  of  snich  sensory  cells  occur  in  pits  or 
longitudinal  grooves  along  the  sides.     These  are  called  the  or- 
gans of  the  lateral  line.     Their  exact  function  is  still  in  some 
doubt.     They  may  possibly  assist  in  the  determination  of  the 
chemical  condition  of  the  water  or  in  determining  the  equilib- 
rium of  the  animal  in  the  water. 

372.  The  Chemical  Senses — Taste  and  Smell. — The  chem- 
ical senses  involve  close  contact  and  a  chemical  union  between 
the  substance  to  be  perceived  and  the  organ  itself.     For  that 
reason  the  substance  must  be  capable  of  solution  in  the  fluids 
that  moisten  the  surfaces.     In  vertebrates  these  organs  are 
located  at  the  anterior  end  of  the  body  and  usually  within  special 
pits  or  cavities.     The  taste  organs  are  in  the  mouth,  especially 
on  the  tongue  and  soft  palate.     In  some  animals  the  sense  is 
poorly  developed.     The  end  organs  of  the  sense  of  smell  are 
located  in  pits  (nose),  anterior  or  dorsal  to  the  mouth,  lined 
with  folds  of  the  mucous  epithelium.     In  most  fishes  these  pits 

23 


354  ZOOLOGY 

are  not  connected  with  the  pharynx,  but  in  all  air-breathing 
forms  there  is  such  connection,  and  the  nostrils  constitute  the 
normal  inlet  for  air  to  enter  the  lungs.  The  sense  of  smell  is 
much  more  developed  in  the  air-breathing  vertebrates,  if  indeed 
it  can  be  said  to  exist  at  all  in  the  aquatic  animals.  It  is  not 
always  easy  to  distinguish  between  smell  and  taste.  The  ol- 
factory organs  are  more  sensitive  than  those  of  taste.  At  any 
rate  smaller  particles  of  the  stimulating  substances  will  arouse 
the  sense  of  smell  than  will  serve  to  arouse  taste.  This  makes  it 
true  that  smell  is  useful  to  detect  chemical  conditions  at  a  greater 
distance  than  is  possible  in  taste. 

373.  The  Ear. — There  is  a  single  pair  of  ears  in  verte- 
brates, and  these  are  located  at  the  sides  of  the  head  behind  the 
eyes.  The  essential  sensory  portion  of  the  ear  (internal  ear) 
arises  as  an  in-pocketing  of  ectoderm,  and  consists  of  a  closed, 
fluid-filled  membranous  sac  which  is  surrounded  by  meso- 
dermal  structures — often  solid  bone.  Ordinarily  this  sac  is  con- 
stricted, being  thus  partially  separated  into  two  irregular  cham- 
bers, the  dorsal  (utriculus)  and  the  ventral  (sacculus).  From 
the  former  arise  three  semicircular  canals  which  are  supplied 
with  sensory  hair-cells  in  the  epithelial  lining  and  are  looked 
upon  as  being  an  organ  to  assist  in  detecting  direction  of  motion 
and  maintaining  balance  or  equilibrium.  From  the  sacculus 
arises  an  outgrowth,  the  cochlea,  which  in  higher  forms  is  well 
developed.  It  becomes  spiral,  and  is  well  supplied  with  sensory 
cells.  It  is  regarded  as  the  chief  auditory  organ  in  those  forms 
possessing  it.  This  membranous  sac  or  labyrinth  is  completely 
surrounded  by  cartilage  or  bone  in  fishes,  and  lies  toward  the 
middle  line  from  the  spiracle.  There  is  no  external  ear.  In 
forms  above  the  fishes  a  membrane  (tympanic  membrane)  stops 
up  the  spiracle  and  incloses  what  is  known  as  the  middle  ear, 
which  still  communicates  with  the  mouth  by  the  Enstachian 
canal.  A  bridge  of  minute  bones  is  also  formed  from  the  tym- 
panic membrane  across  the  middle  ear  whereby  the  external 
vibrations  can  be  communicated  to  the  internal  ear.  In  addi- 
tion to  this,  particularly  among  the  mammals,  is  found  an  exter- 
nal tube  leading  to  the  tympanic  membrane.  Expanded  folds 


CHORDATA 


355 


of  skin  supported  by  cartilage  form  a  funnel  to  catch  the  waves. 
The  tube  (external  auditory  meatus)  and  the  funnel  or  pinna 
constitute  the  external  ear. 

374.  The  Eye. — The  eyes  of  vertebrates  are  a  single  pair  of 
organs  lying  imbedded  in  an  orbit  of  cartilage  or  bone,  within 
which  they  have  considerable  freedom  of  motion.  Six  muscles, 

FIG.  175- 


FIG.  175.  Diagrammatic  horizontal  section  through  the  right  eye  of  Man.  The  line  a  p  is  the 
axis  of  vision.  The  optic  nerve  leaves  the  eye  on  the  median  side  of  this  line.  a.c.t  central  artery; 
a. h.,  aqueous  humor;  &.,  blind  spot,  the  entrance  of  the  optic  nerve;  c,  conjunctiva;  ch.,  choroid 
layer  of  the  eyeball;  c.L,  crystalline  lens;  c.m.c.,  circular  fibres  of  the  ciliary  muscles;  c.m.r.,  radial 
fibres  of  the  ciliary  muscles;  co.,  cornea,  the  transparent  portion  of  the  sclerotic;  c.p.,  ciliary  process; 
c.s.,  canal  of  Schlemm,  a  lymphatic  vessel;  fo.,  fovea  centralis,  the  point  of  clearest  vision;  o.«., 
optic  nerve;  0.5.,  ora  serrata,  the  anterior  wavy  margin  of  the  visual  portion  of  the  retina;  r,  the 
retinal  layer;  sc.t  sclerotic  layer;  sh.,  sheath  of  the  optic  nerve;  v.h.,  vitreous  humor. 

Questions  on  the  figure. — Which  is  the  essential  sensory  portion  of  the  eye? 
Which  parts  are  concerned  in  bringing  the  rays  of  light  to  a  focus?  How  many 
refractive  surfaces  are  present?  How  many  refractive  media?  Which  portions 
of  the  eye  are  primarily  protective  and  supportive?  What  is  the  function  of  the 
various  parts  of  the  choroid  layer?  In  what  way  is  an  image  formed  on  the  retina, 
of  objects  in  front  of  the  lens? 

four  straight  (rectus)  and  two  oblique,  serve  to  move  the  eyeball. 
These  muscles  receive  the  third,  fourth,  and  sixth  of  the  cranial 
nerves.  In  the  higher  forms  muscular  folds  of  the  skin  serve 
to  protect  the  eye  in  front.  The  upper  and  lower  lids  act 
vertically,  but  the  third  (nictitating  membrane)  works  from  the 


356 


ZOOLOGY 


inner  angle  of  the  eye  outward.     Sometimes  all  three  lids  are 
present  together.     In  the  lower  groups  the  lids  are  wanting. 

The  essential  part  of  the  eye  is  the  sensory  expansion  of  the 
optic  nerve — the  retina — which  occupies  the  innermost  posi- 

PIG.  176. 


PIG.  176.  Diagram  showing  some  of  the  retinal  elements  in  their  relations  to  each  other. 
Layer  I  is  directed  toward  the  interior  of  the  eye  and  consists  of  nerve  fibres  (/)  which  finally  enter 
the  optic  nerve  at  the  blind  spot;  2,  the  ganglion-cell  layer,  made  up  of  the  nerve  cells  of  which  the 
fibres  are  a  part;  3.  the  inner  "molecular  layer"  made  up  of  the  fine,  much-branched  nerve  fibrils 
from  2  and  4;  4,  the  inner  nuclear  layer,  containing  numerous  ganglion  cells  (g) ;  5,  the  outer  "mo- 
lecular layer,"  similar  in  structure  to  3;  6,  outer  nuclear  layer,  containing  the  nuclei  of  the  rod  and 
cone  cells  (r.c.  and  c.c.);  7,  the  layer  of  rods  and  cones  (r,  c).  This  is  the  nervous  epithelium,  or 
the  nerve-endings  of  vision.  The  rods  and  cones  are  partly  imbedded  in  a  pigment  epithelium  (8). 
It  must  be  remembered  that  hundreds  of  elements  are  omitted  where  one  is  shown  in  the  figure. 

Questions  on  the  figure. — Which  portion  of  the  retina  does  the  light  first 
strike  on  entering  the  eye  ?  To  what  point  must  it  penetrate  to  arouse  nervous 
activity  ?  Over  what  route  must  a  nervous  impulse  pass  to  reach  the  brain  from 
the  point  of  stimulation  (rods  and  cones)  ?  Compare  with  similar  figures  in  other 
texts. 


tion,  bounding  the  posterior  portion  of  the  cavity  of  the  eyeball. 
This  is  a  very  complicated  layer,  but  a  general  idea  of  it  can  be 
obtained  from  the  diagram  (Fig.  176).  The  layer  of  rods  and 
cones,  in  close  connection  with  a  layer  of  pigment,  is  the  sensi- 


CHORD ATA  357 

tive  layer.  Light  falling  directly  on  this  from  all  directions 
would  produce  no  image,  just  as  the  photographic  plate  exposed 
outside  the  camera  would  present  a  general  blur.  We  find  then 
the  necessity  of  the  same  optical  devices  as  are  found  in  the 
camera:  (i)  a  sensitive  surface,  the  retina;  (2)  a  box  for  sup- 
port and  for  keeping  out  the  light  except  from  one  direction, 
the  opaque  layers  of  the  ball  of  the  eye;  (3)  an  aperture  for  the 
passage  of  the  light  into  the  interior  of  the  box,  the  pupil  and 
the  transparent  cornea  overlying  it;  and  especially  (4)  a  lens 
or  series  of  refracting  surfaces  which  cause  all  the  rays  of  light 
coming  to  the  eye  from  each  external  point  to  be  brought  to- 
gether again  beyond  the  lens  at  a  corresponding  point  on  the 
sensitive  surface.  The  elementary  relations  of  these  parts 
as  found  in  the  eyes  of  vertebrates  may  be  gathered  from  a 
study  of  Fig.  175. 

Accommodation  of  the  eye  to  objects  at  different  distances 
is  effected  by  means  of  changes  in  the  shape  of  the  lens  through 
the  action  of  appropriate  muscles. 

375.  Library  Exercise. — What  portions  of  the  vertebrate 
eye  are  derived  directly  from  the  ectoderm?     Which  from  the 
brain   (i.e.,  indirectly  from    ectoderm)?      Which  from    meso- 
derm ?     (See  Fig.  45.) 

What  variation  occurs  among  vertebrates  as  to  the  condi- 
tion of  the  bones  in  the  middle  ear?  Whence  are  they  con- 
sidered to  be  derived  ?  What  variation  in  the  cochlea  ?  Study 
from  figures  the  structure  of  the  cochlea. 

376.  Classification. — The    principal    divisions    of    the    sub- 
phylum  vertebrata  are : 

Class  I.  Pisces  (e.g.,  sharks,  lung-fishes,  bony  fishes). 
Page  3 58. 

Class  II.     Amphibia  (frogs,  toads,  salamanders,  etc.).   Page 

375- 

Class  III.  Reptilia  (crocodiles,  lizards,  snakes,  turtles). 
Page  384. 

Class  IV.     Aves  (birds).     Page  398. 

Class  V.     Mammalia  (mammals).     Page  437. 


CHAPTER  XX 

CLASS  I.— PISCES 

377.  The  class  of  fishes,  representatives  of  which  are  famil- 
iar to  all,  is  important  not  only  from  the  point  of  view  of  its 
specialized  present-day  representatives  but  from  the  fact  that 
it  was  the  first  successful  vertebrate  group  of  geological  times. 
It  represents  the  primitive  aquatic  habit  from  which  the  land- 
inhabiting  types  of  vertebrates  must  have  arisen,   and  in  it 
we  find  the  fundamental  plan  of  structure  which  has  been 
modified  in  the  higher  forms  (as  the  Amphibians)  in  adaptation 
to  their  present  mode  of  life.     It  must  of  course  be  borne  in 
mind  that  the  types  of  fishes  which  are  supposed  to  be  the 
ancestors    of    the    air-breathing    vertebrates    were    much    less 
specialized  in  structure  than  the  present  members  of  the  class. 
There  are,  however,  even  now  some  of  the  fishes  which  have 
changed  less  than  the  majority,  from  the  primitive  condition. 

378.  General  Characteristics.— 

1.  Fishes   are   aquatic  vertebrates  having  gills  functional 
throughout  life.     These  consist  of  filaments  or  sheets,  contain- 
ing blood-vessels  and  attached  to  bony  or  cartilaginous  arches 
in  the  region  of  the  pharynx. 

2.  Paired  appendages   (pectoral  and  pelvic)   are  normally 
fin-like, — not  having  a  median  jointed  axis  as  in  the  limbs  of 
the  higher  vertebrates.     The  medial  fins  are  dorsal,  ventral, 
and  caudal.     The  last  is  the  chief  organ  of  locomotion. 

3.  There  is  usually  a  dermal  skeleton  consisting  of  scales, 
covered  with  epidermis.     The  latter  may  deposit  enamel  on 
the  dermal  core  of  the  scale. 

4.  There  is  a  two-chambered  heart  through  which  the  sys- 
temic (impure)  blood  flows. 

5.  Vertebral  column  either  cartilaginous  or  bony;  the  verte- 
brae biconcave. 

358 


PISCES  359 

6.  There  is  no  true   (allantoic,   see  Fig.   208,   al.)   urinary 
bladder. 

7.  A  longitudinal  line  of  sense  organs  ("organs  of  the  lateral 
line")  on  each  side  of  the  body. 

8.  Nasal  pit  does  not   (usually)   communicate  posteriorly 
with  the  mouth. 

9.  Fertilization  and  development  usually  take  place  outside 
the  body. 

379.  Form. — Fishes  usually  have  a  body  somewhat  flattened 
from  side  to  side,  though  it  may  be  quite  cylindrical  or  flat- 
tened dorso-ventrally,   and  gradually  tapering  toward  either 
end.     This  is  readily  seen  to  be  a  form  well  suited  to  motion 
through  water.     Those  which  enter  crevices  under  rocks  or 
elsewhere,  as  eels  and  catfish,  are  cylindrical.     Some  types  are 
extremely  freakish  in  form,  as  the  spherical  globe  fish,  angler- 
fish  which  is  chiefly  head,  the  strange  hammer-headed  shark, 
the  pointed   gar-pike   and   sword-fish,    and  the  indescribable 
seahorse. 

The  head  end,  while  not  so  specialized  as  in  the  higher  forms, 
is  much  more  cephalized  than  in  the  Protovertebrata.  The 
mouth  with  its  modifications  of  movable  jaws,  teeth,  etc.,  the 
respiratory  arches,  the  sense  organs  of  sight,  hearing,  taste, 
and  possibly  smell,  the  brain  and  brain  case  all  enter  into  this 
cephalization. 

There  is  no  neck,  i.e.,  the  head  is  not  movable  with  refer- 
ence to  the  body.  The  length  of  the  body  varies  very  greatly 
as  does  the  number  of  metameres  embraced.  The  body  may 
be  distinguished  as  pre-caudal  and  caudal. 

380.  Appendages. — Fishes  possess  two  classes  of  append- 
ages— paired,   or  lateral,   and  median.     These  are  expansions 
of  the  skin  supported  by  rays  of  bone  or  cartilage.     The  paired 
fins  are  four  in  number  and  are  considered  to  be  homologous 
with  the  pectoral  and  pelvic  appendages  of  the  higher  verte- 
brates.    They  differ  much  as  to  their  position,  especially  the 
posterior  pair,  as  may  be  seen  by  a  comparison  of  the  figures  in 
this  chapter.     In  its  typical  condition  the  appendage  consists 
of  girdle  and  the  fin  proper.     In  the  lung-fishes  there  is  a  central 


ZOOLOGY 


axis  (Fig.   177)   through  the  fin,  instead  of  the  usual  radiate 
arrangement  (Fig.  178,  J.r.)  of  the  fin-rays.     The  legs  of  higher 

FIG.  177. 


FIG.  177.    Diagram  of  the  anterior  fin  (archipterygium)  of  Ceratodus. 

Questions  on  the  figure. — What  are  the  chief  points  of  contrast  between  this 
fin  and  that  of  Teleosts,  figured  in  Fig.  178?  How  does  Gegenbauer  consider  that 
this  might  give  rise  to  the  simpler  type  of  Vertebrate  legs? 

FIG.  178. 


FIG.  178.  Pectoral  girdle  and  fin  of  a  Teleost.  br.o.t  branchial  ossicles;  c,  coracoid;  d., 
cliavcle;  f.r.,  fin  rays;  p.cl.,  post  clavicle;  p.t.,  post  temporal  which  unites  with  skull;  sc.t 
•capula;  s.cl.,  supra-clavicle.  By  Folsom. 

Questions  on  the  figure. — Which  girdle  is  this,  right  or  left?  Do  authors 
regard  any  of  these  bones  as  homologous  with  similarly  named  bones  in  the  higher 
Vertebrates? 

vertebrates  are  supposed  to  have  been  derived  from  this  type 
the  archipterygium). 


PISCES  301 

The  median  fins  consist  of  one  or  more  dorsal  portions  which 
vary  in  extent,  a  caudal  portion,  and  a  ventral  part  near  the 
anus.  These  may  represent  remnants  of  a  continuous  median 
fin  such  as  is  seen  in  Amphioxus  (Fig.  154).  They  are  sup- 
ported by  fin-rays  in  the  dermal  fold,  which  are  in  turn  sup- 
ported by  spines  imbedded  in  the  muscles.  The  form  of  the 
caudal  fin,  which  is  much  used  in  locomotion,  differs  widely. 

FIG.  179. 


FIG.  179.  Diagrams  of  some  principal  forms  of  tails  in  fishes.  A,  protocercal  fin  (as  in  Polyp- 
erus) ;  B,  heterocercal  (as  in  Sharks) ;  C,  homocercal  (as  in  most  Teleosts) ;  D,  homocercal  (as  in 
Amia).  By  Polsom. 

Questions  on  the  figures. — What  is  the  essential  difference  between  the  sym- 
metry of  D  and  of  A  ?  What  conceivable  advantage  has  the  symmetrical  over  the 
unsymmetrical  type?  Are  the  heterocercal  types  successful  swimmers? 

These  differences,  correlated  with  modifications  of  the  end  of 
the  vertebral  column,  have  considerable  importance  in  subdivid- 
ing the  class.  The  following  types  may  be  noted: 

1.  The  vertebral  column  passes  straight  to  the  end  of  the 
tail  and  the  fin-rays  are  disposed  symmetrically  with  regard 
thereto  (protocercal) ;  found  in  lung-fishes  and  some  primitive 
extinct  forms  (Fig.  179,  ^4). 

2.  The  vertebral  column  is  bent  dorsad,  and  a  small  fin  lobe 
develops  from  its  ventral  side.     The  tail,  though  two-pronged, 
is  not  symmetrical  (heterocercal).     Found  in  sharks  and  many 
ganoids  (Fig.  179,  B). 

3.  The  vertebral  column  may  become  still  more  bent  and 


362  ZOOLOGY 

reduced;  the  ventral  lobe  develops  until  the  whole  structure 
appears  symmetrical  again  (homocercal) .  Found  in  bony  fishes 
(Fig.  179,  C,  D). 


FIG.  180.  Skull  of  Cod  (Gadus  morrhua).  From  Nicholson,  after  Owen,  b,  branchiostegal 
rays  borne  on  c.h.,  the  ceratohyal  bone;  d,  dentary  portion  of  the  mandible;/,  frontal;  h.m.,  hyoman- 
dibular;  i.o.,  interoperculum;  /,  lachrymal;  m,  maxilla;  n,  nasal;  o,  operculum;  p.m.,  premaxilla; 
P.O.,  preoperculum;  p.s.,  parasphenoid;  q,  quadrate;  s.o.,  sub-operculum;  s.oc.,  supra-occipital. 

Questions  on  the  figure. — What  is  the  operculum?  How  many  bones  are 
associated  to  form  it?  Which  bones  are  figured  as  bearing  teeth?  Which  of 
these  bones  belong  to  the  cranium  proper  ?  What  is  the  difference  between  cranium 
and  skull?  What  do  authors  believe  to  be  the  origin  and  homology  of  the  chief 
facial  bones? 

381.  Covering. — Most  fishes  are  more  or  less  covered  by 
scales  or  scutes  of  bony  matter  developed  in  the  dermis  and 
lying  between  the  dermis  and  epidermis.  The  scales  often 
receive  a  layer  of  enamel  from  the  epidermis.  In  form  they 
may  be  cycloid  (round,  with  smooth  margin),  ctenoid  (toothed 
margin),  placoid  (plate-like  bodies  often  bearing  points  covered 
with  enamel),  and  ganoid  (thick  rhomboid  scales  covered  over 
with  enamel,  and  often  closely  articulated  into  a  coat-of -armor). 
The  scales  are  usually  placed  as  shingles  are  on  a  roof,  and  doubt- 
less protect  the  fish  from  mechanical  injuries.  A  good  many 
species  of  fishes  are  destitute  of  scales  altogether,  the  skin  of 
such  often  being  supplied  with  numerous  mucous  glands.  In 


PISCES  363 

many  extinct  forms  the  external  covering  was  made  up  of  large 
plates  fused  into  a  dense  armor. 

382.  The    Skull. — The   skull   in   fishes   is   especially   note- 
worthy for  the  looseness  of  the  connection  between  the  facial 
bones  (i.e.,  the  visceral  or  branchial  arches)  and  the  cranium. 
They  are  readily  separated  from  the  cranium.     The  lower  jaw 
is  not  articulated  directly  with  the  brain-case  but  with  the  upper 
jaw  (see  Fig,  180,  q). 

383.  Locomotion. — Fishes    are    aquatic    and    are    complete 
masters  of  their  medium.     The  density  of  water  as  compared 
with  air  makes  the  matter  of  support  in  the  medium  much 
easier  for  the  fish  than  for  the  bird.     The  denser  medium  is  how- 
ever more  difficult  to  penetrate.     The  specific  gravity  of  the 
fish  as  a  whole  does  not  widely  differ  from  that  of  water,  although 
it  varies  within  narrow  limits.     Four  problems  are  thus  pre- 
sented to  the  fish  for  solution : 

1.  The  Regulation  of  Specific  Gravity. — This  is  effected  in 
part  at  least  by  the  air  bladder.     The  body  muscles  may  bring 
about  the  compression  of  the  contained  gas  and  thus  decrease 
the  size  without  change  of  weight. 

2.  Propulsion. — The  chief  organ  of  propulsion  is  the  caudal 
fin,  acted  on  by  the  powerful  lateral  muscles  of  the  body.     The 
resultant  of  the  alternate  strokes  against  the  water  is  forward 
motion.     This  may  be  supplemented  by  the  action  of  the  paired 
fins,  especially  in  slow  motion. 

3.  Steering. — This  is  accomplished  in  part  by  the  changes 
in  specific  gravity  and  the  regulation  of  the  stroke  of  the  tail,  and 
in  part  by  the  action  of  the  paired  fins.     The  semicircular  canals 
probably  assist  the  animal  in  appreciating  changes  in  its  posi- 
tion,— its  orientation,  thus  enabling  it  to  choose  its  direction. 

4.  Balancing. — Since  most  fishes  are  flattened  from  right  to 
left  there  is  some  difficulty  in  keeping  an  upright  position.     The 
sense  of  position  is  doubtless  given  by  the  semicircular  canals. 
The  paired  fins  are  used  in  balancing,  as  are  the  dorsal  and  anal. 
Doubtless  all  the  fins  along  the  body  of  the  fish  are  used  some- 
what as  the  keel  of  a  boat  or  the  planes  of  an  air  ship  or  a 
submarine. 


364 


ZOOLOGY 


c.v.r; 


c.v.l. 


FIG.  181.  Diagram  of  the  principal  vessels  in  the  circulation  of  a  fish, — ventral  view,  a, 
•orta;  OH.,  auricle;  br.a.,  afferent  branchial  arteries;  br.e.,  efferent  branchial  arteries;  bit.,  bulbus 
(or  conus)  arteriosus;  c,  carotid;  c.a.,  caudal  artery;  c.v.r.,  right  cardinal  vein;  £.»./.,  left  cardinal 
vein;  d.a.,  dorsal  artery;  d.c.,  ductus  Cuvieri;  g,  gills;  h.v.,  hepatic  vein;  h.p.,  hepatic  portal  vein;  », 
intestine;  j,  jugular  vein;  k,  kidney;  /,  liver;  r.p.,  renal  portal  vein;  s.v.,  sinus  venosus;  v,  ventricle. 

Questions  on  the  figure. — Follow  the  general  course  of  the  circulation,  noting 
changes  in  the  character  of  the  blood  in  various  capillary  regions.  What  is  the 
extent  of  the  hepatic  portal  system?  Of  the  renal  portal?  Where  is  the  purest 
blood  in  the  body?  Reason  for  your  answer?  What  do  you  mean  by  "impure" 
blood?  Where  are  the  chief  impurities  removed  from  the  circulation? 


PISCES  365 

/ 

384.  Supplementary  Exercise  for  the  Library. — What  is  the  structure  and 
position  of  the  "swim-"  or  air-bladder  in  fishes?  With  what  organ  is  it  related? 
Does  it  communicate  with  the  outside?  Are  there  any  evidences  that  it  is  of 
value  as  a  respiratory  organ  in  any  of  the  fishes?  Can  you  conceive  any  use  it 
might  be  in  steering,  for  the  purpose  of  rising  or  sinking  in  the  water?  What 
would  be  the  effect  of  compressing  the  air-bladder  at  one  end  more  than  at  the 
other? 

385.  The  Circulation. — Little  needs  be  said  here  in  addition 
to  what  has  been  said  in  the  general  discussion  of  the  vertebrate 
circulation  (see  Figs.  166-171).  The  heart  is  two-chambered. 
The  auricle  receives  the  venous  blood  from  the  system;  it  is 
passed  to  the  ventricle  through  a  valve  which  forbids  its  passage 
in  the  reverse  direction.  From  the  ventricle  the  blood  passes 
through  a  valvular  region  into  the  ventral  aorta,  which  carries  it, 
by  a  series  of  right  and  left  branches,  to  the  gills.  Here  aeration 

FIG.  182. 

ci/Z 


FIG.  182.     Diagram  of  the  principal  vessels  in  the  circulation  of  a  Fish, — lateral  view.     Lettering 
as  in  the  preceding  figure.     Adapted  from  Parker  and  Haswell. 

Questions  on  the  figure. — Compare  the  two  views  (Figs.  181  and  182)  and 
identify  the  parts  common  to  both,  tracing  the  course  of  the  circulation  in  the 
various  vessels. 

takes  place,  the  pure  blood  being  gathered  from  the  gills  by  a 
series  of  efferent  branches  which  combine  (except  some  anterior 
branches  which  go  to  the  head)  to  form  a  dorsal  aorta.  The 
dorsal  aorta  gives  off  branches  to  the  body  wall,  to  the  paired 
appendages,  to  the  liver,  digestive  tract  and  kidneys, — con- 
tinuing into  the  tail  where  it  breaks  up  in  the  muscles. 

The  impure  blood  from  the  capillaries  of  the  tail  is  brought 
back  to  the  kidneys  by  the  renal  portal  vein,  where  it  again 
passes  through  capillaries;  here  the  blood  is  purified  of  its  urea 


366  ZOOLOGY 

and  similar  impurities.  The  blood  supplied  direct  to  the  kid- 
neys from  the  aorta  and  that  of  the  renal-portal  circulation  is 
returned  to  the  heart  by  way  of  right  and  left  (cardinal)  veins 
which  join  corresponding  right  and  left  veins  (jugular)  from  the 
head  to  form  the  veins  (ductus  Cuvieri)  which  empty  into  the 
auricle.  The  blood  which  was  distributed  to  the  stomach  and 
intestines  is  gathered  into  a  vessel  (hepatic  portal  vein)  which 
carries  it  to  the  liver,  together  with  much  of  the  food  absorbed 
from  the  intestines.  The  hepatic  portal  vein  here  breaks  up 
into  capillaries.  The  blood  from  the  liver  and  from  the  appen- 
dages unites  with  that  carried  by  the  ductus  Cuvieri  before  it 
reaches  the  heart.  The  student  should  carefully  follow  out  the 
course  of  the  circulation  in  the  accompanying  diagrams  (Figs. 
181,  182)  and  determine  just  what  changes  take  place  in  the 
blood  in  the  various  capillaries  through  which  it  passes.  Varia- 
tions from  this  typical  condition  are  numerous,  accounts  of 
which  must  be  sought  in  more  extended  texts. 

386.  Library  Exercise. — Find  description  and  figures  in  the  reference  zoologies 
locating  the  unfamiliar  structures  in  circulation  of  fishes  in  the  table  on  page  343 
and  testing  the  statements  found  there. 

387.  Habits  and  Food. — Fishes  occur  abundantly  both  in 
fresh  and  salt  water.  Usually  the  whole  life  is  spent  under  the 
same  general  conditions.  The  salmon  and  shad,  however,  are 
bred  and  partly  develop  in  fresh  water  and  later  pass  out  to  the 
sea.  These  forms  return,  often  with  remarkable  precision,  to 
the  place  of  their  own  hatching  to  deposit  their  eggs.  Use  is 
made  of  this  habit  in  the  capture  of  them  for  commercial  pur- 
poses. Unless  some  means  are  found  for  limiting  the  destruc- 
tion of  the  adults  during  the  breeding  period,  some  of  our  most 
valuable  food  fishes  are  in  danger  of  extermination.  Others,  as 
the  eels, -may  generate  in  the  sea  and  spend  a  part  or  all  their 
adult  life  in  fresh  water.  Some  burrow  in  the  muddy  bottoms, 
as  the  eel,  cat-fish,  mud-fish;  others  lie  on  the  bottom,  as  the 
flat-fish;  many  quaint  forms  frequent  the  depths  of  the  sea. 
The  most  are  active  swimmers  in  open  water  near  or  away  from 
the  shore.  Many  forms  (herring,  shad,  salmon,  etc.),  are  dis- 
tinctly gregarious,  moving  in  great  shoals  especially  at  spawning 
time  or  when  in  search  of  food.  This  fact  and  the  knowledge 


PISCES  367 

of  places  and  times  are  matters  of  much  moment  to  the  fisher- 
men. The  food  of  fishes  is  very  diverse.  Some  forms  are  ac- 
tively carnivorous,  preying  on  animals  as  large  or  larger  than 
themselves  (sharks);  others,  and  these  are  the  most  numerous 
class,  depend  upon  small  animals  such  as  the  young  of  their  own 
or  other  species  of  fish,  on  Crustacea,  insects  and  worms.  The 
microscopic  animals  and  plants  occurring  in  immense  numbers 
in  the  water  are  important  items  in  the  food  of  fishes.  Some 
fishes  are  scavengers,  living  largely  upon  the  dead  materials 
found  in  the  water.  Fishes  differ  much  in  their  energy,  courage, 
and  resistance  to  attack.  Those  possessing  these  qualities  in 
high  degree  are  denominated  "game"  fish  and  are  prized  for  the 
difficulty  involved  in  their  capture.  The  family  of  the  trout 
and  salmon  includes  several  such  species. 

The  deep  sea  habit  results  in  most  interesting  adaptations. 
The  conditions  of  life  are  different  from  anything  we  know. 
The  darkness  is  complete,  the  temperature  invariable  and  not 
far  above  freezing  point,  the  pressure  is  something  enormous. 
Many  of  the  fishes  are  phosphorescent.  Sometimes  these 
luminous  organs  seem  arranged  to  attract  the  prey.  Some 
forms  have  very  large  eyes  to  use  the  weak  phosphorescent 
light ;  others  are  blind  as  are  some  of  the  cave  fishes.  The  sense 
of  touch  is  greatly  developed.  In  form  they  are  very  varied 
and  bizarre. 

388.  Economic  Value. — From  primitive  times  fish  has  been 
one  of  the  important  human  foods.  Probably  a  larger  percent- 
age of  the  well-known  species  of  fishes  are  regarded  as  edible 
than  of  any  other  animal  group.  Their  rate  of  multiplying 
and  their  occurrence  in  schools  at  available  points  are  quite  as 
important  factors  as  the  delicacy  of  the  flesh  in  determining  the 
food  value  of  a  species.  The  improved  devices  for  capturing 
fish,  the  development  of  methods  of  preserving  them  by  drying 
and  by  canning,  and  the  increased  price  of  other  food  substances 
for  which  fish  may  be  substituted  have  all  conspired  to  increase 
the  destruction  of  the  more  important  edible  fish  both  in  the 
fresh  and  salt  waters.  In  recognition  of  this,  most  nations  have 
appointed  commissions  for  the  study  of  problems  connected  with 


368 


ZOOLOGY 


the  fisheries  and  for  the  better  regulation  of  the  same.  The 
United  States  Fish  Commission  in  conjunction  with  similar 
state  boards,  has  done  an  immeasurable  amount  of  good  espe- 

FIG.  183. 


FIG.  183.     Atlantic  Salmon  (Salmo  salar).     From  the  "Manual  of  Fish  Culture,"  U.  S.  F.  C. 

Questions  on  the  figure. — What  are  the  names  of  the  various  fins  shown  in  the 
figure?  What  is  the  dotted  line  along  the  side  of  the  fish?  What  type  of  tail 
has  this  fish?  . 

FIG.  184. 


FIG.   184.     Brook  Trout  (Salvelinus  fontinalis') .     From  "  Manual  of  Fish  Culture,"  U.  S.  F.  C 

daily  in  the  following  particulars : 

i.  In  taking  the  spawn  of  our  most  important  food  fishes 
and  caring  for  it  artificially  during  the  period  of  early  develop- 
ment when  the  young  animals  are  in  the  greatest  danger  of 


PISCES  369 

destruction.  Such  fish  hatcheries  are  scattered  all  over  the 
Union  and  many  of  our  fresh  waters  are  being  restocked  with 
species  believed  to  be  hardy  and  suitable  for  food. 

2.  By  determining  the  foods  preferred  by  special  fish  and 
artificially  encouraging  its  abundance. 

3 .  By  encouraging  the  destruction  of  species  of  animals  that 
prey  upon  the  food  fishes,  and  by  the  study  of  fish  diseases  pro- 
duced by  plant  and  animal  parasites. 

4.  By  studying  the  habits  of  the  fishes  and  by  regulation  of 
the  time,  place  and  manner  of  catching. 

The  money  value  of  the  annual  catch  of  fish  in  our  waters 
cannot  well  be  less  than  $50,000,000.  The  most  valuable  fresh- 
water forms  are  the  lake  trout,  white  fish,  cat  fish,  bass,  perches, 
suckers.  The  chief  marine  species  are  cod,  haddock,  halibut, 
mackerel,  menhaden,  herring,  and  salmon.  The  latter,  though 
marine,  is  caught  in  fresh  waters  in  its  breeding  migrations. 

There  are  numerous  species  of  fish  which  are  destructive  to 
the  food  fishes,  by  devouring  the  young  or  in  other  ways. 
Among  these  are  the  gar-pike,  pickerel,  muscalonge,  German 
carp,  the  dogfish  and  other  sharks. 

389.  Supplementary  Exercises  for  the  Library. 

1.  Make  a  report  concerning  the  principal  food  fishes  used  by  the  people  of  the 
United  States:  their  habits  and  geographical  range,  the  mode  of  their  capture  and 
putting  on  the  market. 

2.  Make  a  study  of  the  methods  of  capturing  fish  from  primitive  time  to  the 
present  and  show  how  the  methods  have  been  adapted  to  the  habits  of  the  fish. 

3.  A  study  of  the  history  and  work  of  the  United  States  Fish  Commission  as 
shown  in  the  annual  reports  and  bulletins.     Its  economic  side.     Its  scientific  side. 

390.  Reproduction  and  Development. — The  sexes  are  sepa- 
rate. The^  sexual  elements  are  produced  in  great  numbers. 
The  ova  (spawn)  are  usually  deposited  in  the  water,  in  shallows 
on  the  open  bottom,  under  rocks,  or  in  places  specially  provided 
for  them  by  the  parents.  The  sperm  (milt)  is  poured  over  these 
by  the  male,  and  the  fertilization  and  later  development  take 
place  in  the  water  with  little  or  no  care  on  the  part  of  the  parents. 
Great  loss  of  life  occurs  among  the  young  from  the  voracious 
habits  of  other  species  and  sometimes  of  the  parents  themselves. 
It  is  not  difficult  to  believe  that  the  enormous  number  of  eggs 
produced  by  the  female  is  an  adaptation  to  meet  this  risk  of 
24 


370  ZOOLOGY 

mortality  among  the  young.  In  some  cases  (most  sharks  and 
a  few  bony-fishes)  the  eggs  are  fertilized  and  the  young  hatched 
within  the  body  of  the  mother.  Only  a  few  young  are  produced 
in  such  forms. 

The  eggs  of  fishes  are  usually  well  supplied  with  yolk,  seg- 
mentation being  partial  (discoidal,  see  §53).  The  unsegmented 
portion  comes  to  be  surrounded  by  a  yolk  sac  and  furnishes 
nourishment  for  the  early  stages  of  development. 

FIG.  185. 


FlG.   185.     The  Smelt  (Osmerus  dentex}.     Bull.  U.  S.  Fish  Commission. 

391.  Special  Adaptations. — In  addition  to  those  already 
mentioned  the  group  of  fishes  shows  many  adaptations  to  special 
modes  of  life. 

Color. — Most  fishes  show  color  as  the  result  of  pigment 
buried  in  the  cells  of  the  skin,  or  of  delicate  markings  on  the 
scales.  In  general,  the  tone  of  color  accords  with  the  environ- 
ment. This  becomes  very  striking  in  some  of  the  less  active 
forms,  as  the  flounders,  in  which  the  colors  may  change  more  or 
less  rapidly  to  accord  with  the  bottom  on  which  they  lie.  It 
seems  probable  that  some  degree  of  protection  from  enemies 
may  thus  be  gained,  which  would  be  of  distinct  value  to  the 
species.  In  some  cases  the  color  is  in  sharp  contrast  with  the 
environment,  and  may  be  very  conspicuous.  This  is  believed 
to  be  a  warning  coloration  in  some  instances,  accompanying 
some  disagreeable  quality.  Some  deep-sea  forms  are  phos- 
phorescent. This  is  probably  of  considerable  importance  ,as  no 
sunlight  penetrates  to  that  depth. 

Electrical  Organs. — In  several  groups  of  fishes  (rays,  eels,  etc.), 
certain  muscular  tracts  have  become  so  modified  that  under 
nervous  stimulus  instead  of  producing  motion  by  contraction 


PISCES  371 

they  form  and  accumulate  electrical  energy  which  may  be  dis- 
charged at  the  will  of  the  animal.  This  power  certainly  has  a 
protective  value,  as  the  discharge  is  in  some  cases  powerful 
enough  to  paralyze  much  larger  animals  than  the  fish  itself.  It 
is  probably  useful  also  in  capturing  prey. 

FIG.  1 86. 


FIG.   186.     Young  Sea-bass  (Centropristis  strialus).     Photo  from  life  by  Dr.  R.  W.  Shufeldt. 

Question  on  the  figure. — Locate  the  pelvic  fin  and  compare  with  other  fish  as  to 
position. 

Asymmetry. — In  the  flat-fishes  we  find  a  very  striking  com- 
pression from  side  to  side.  In  early  life  they  have  the  position 
normal  to  other  fish,  but  in  the  adult  stage  they  rest  and  swim 
with  the  dorso-ventral  plane  horizontal  instead  of  vertical — on 
the  left  side  in  some  species  and  on  the  right  in  others.  The  side 
that  is  uppermost  becomes  pigmented  like  the  back,  and  the 
under  side  loses  its  pigment  and  becomes  white,  as  the  belly  of 
fishes  in  the  normal  position.  The  eye  which  belongs  to  the 
under  side  changes  its  position  until  it  comes  to  lie  on  the  upper 
side.  The  bones  of  the  cranium,  especially  those  about  the  eye, 
are  twisted  and  the  right  and  left  branches  of  the  jaw  are  un- 
equally developed.  The  dorsal  and  ventral  fins  become  con- 
tinuous and  the  body  tends  to  become  bilaterally  symmetrical 
in  the  new  position.  We  can  scarcely  doubt  that  this  asym- 
metrical condition  has  been  brought  about  by  the  position 
which  the  animal  takes  in  relation  to  the  environment,  but  we 
know  that  in  some  species  the  eye  begins  to  migrate  now  before 
the  fish  assumes  the  lateral  position. 


372  ZOOLOGY 

392.  Classification  of  Fishes. 

Subclass  I.  Elasmobranchii  (Sharks,  Dog-fishes,  Rays,  Skates). — Marine 
fishes  with  essentially  cartilaginous  skeleton;  no  operculum  or  gill-cover;  mouth  on 
the  ventral  surface  of  the  head;  heterocercal  tail;  external  skeleton  of  placoid 
scales;  spiral  valve  in  the  intestine;  no  air  bladder.  The  elasmobranchs  are 
regarded  by  some  as  being  the  nearest  present  relatives  of  the  primitive  fishes. 
They  occur  most  abundantly  and  are  larger  individually  in  warm  seas.  They  are 
powerful  swimmers  as  befits  carnivorous,  preying  animals.  They  feed  on  Crustacea, 
Mollusca,  and  fish. 

Subclass  II.  Ganoidei  (Ganoid  Fishes:  Sturgeon,  Gar-pike). — Fishes^with 
bony  cartilaginous  skeleton;  gills  covered  by  an  operculum;  exoskeleton  of  ganoid 
scales  or  enameled  plates;  air-bladder  present;  spiral  intestinal  valve;  tail  either 
homo-  or  heterocercal. 

The  group  was  very  important  in  the  early  history  of  the  earth,  and  the  present 
species  are  a  mere  remnant  of  the  former  glory  of  the  ganoids.  They  occur  now 

FIG.  187. 


FIG.    187.     Long-eared  Sunfish  (L»pomis  auritus).      Adult.      Photo  from  life  by  Dr.  R.  W.  Shufeldt. 

chiefly  in  the  rivers  and  lakes,  though  the  sturgeon  is  also  found  in  the  sea.  North 
America  is  as  well  represented  as  any  other  region  in  the  living  species  of  this 
remarkable  group. 

Subclass  III.  Teleostei  (Bony  Fishes). — Fishes  with  well-ossified  skeletons; 
body  covered  with  cycloid  or  ctenoid  scales;  exoskeleton  of  bony  plates  in  the  head 
region  which  become  associated  with  bones  of  the  internal  skeleton  to  form  the 
skull;  mouth  terminal  rather  than  ventral;  gills  covered;  spiral  valve  lacking;  air- 
bladder  usually  present;  homocercal  tail. 

This  subclass  embraces  the  great  majority  of  the  forms  ordinarily  known  as 
fishes.  There  are  estimated  to  be  6,000  or  more  species  of  teleosts,  more  than  2,000 
of  which  inhabit  fresh  water.  The  group  is  variously  divided  by  different  authors 
and  the  student  must  be  referred  to  more  advanced  texts  for  fuller  classification. 
The  principal  orders  are  outlined  below. 

Pneumatic    duct    (from    air-bladder    to    intestine)    open Order  Physostomi. 

(Carp,  cat-fish,  sucker,  salmon,  trout,  shad,  herring,  eel,  etc.) 
Pneumatic  duct  closed. 

Dorsal,  anal,  and  pelvic  fins  spiny  in  front. 

Bones  of  the  pharynx  (branchial  arches)  distinct ...  Order  Acanthopteri. 
(Perch,  sunfish,  mackerel,  stickleback,  silverside,  etc.) 


PJSCES 


373 


Bones  of  the  pharynx  united Order  Pharyngognathi. 

Dorsal,  anal,  and  pelvic  fins  without  spines Order  Anacanthini. 

(Cod-fish,  haddock,  flat-fish,  etc.) 

(Two  other  teleost  orders  of  less  importance,  embracing  some  very  pecu- 
liar forms,  are  the  Plectognathi  (globe-fishes)  and  the  Lophobranchii  (sea 
horses,  Fig.  60;  and  pipe-fishes). 

FIG.   188. 


PIG.   1 88.     Sheepshead.     Greatly  reduced.     Photographed  from  life  by  Dr.  R.  W.  Shufeldt. 

FIG.  189. 


FIG.   189. — Young  of  the  Snowy  Grouper  (Epinephelus  niveatus). 
Shufeldt:  American  Naturalist. 


Photo  from  life  by  Dr.  R.  W. 


Subclass  IV.  Dipnoi  (Lung-fishes).— Fishes  with  a  persistent  notochord  and 
the  internal  skeleton  incompletely  ossified;  soft  cycloid  scales;  spiral  valve  in  the 
intestine,  the  swim-bladder  used  as  a  lung,  the  auricle  partly  separated  into  two 
chambers,  paired  appendages  with  a  central  axis  producing  a  flapper  rather  than 
a  fin  (Fig.  1 77).  There  are  only  three  or  four  living  species,  but  these  are  especially 


374  ZOOLOGY 

interesting  to  the  zoologist  from  the  fact  that  they  may  represent  the  division  of 
fishes  from  which  the  air-breathing  vertebrates  sprang.  One  genus  (Ceratodus)  is 
found  in  the  rivers  of  Queensland ;  the  second  (Protopterus)  in  the  rivers  of  southern 
Africa,  and  a  third  (Lepidosiren)  in  the  Amazon  in  South  America.  No  marine 
forms  are  known.  From  fossil  remains  it  is  evident  that  the  ancestors  of  the 
present  lung-fishes  were  very  much  more  widely  distributed. 

393.  Supplementary  Studies  for  Library  and  Field. 

1.  What  are  the  theories  as  to  the  origin  of  the  paired  fins  of  fishes? 

2.  In  what  way  do  fishes  change  their  long  axis  from  the  horizontal  position 
so  as  to  ascend  or  descend  obliquely  in  swimming  ? 

3.  Range  of  size  in  fishes. 

4.  Probable  origin  of  fresh-water  fishes.     What  forms  are  now  able  to  pass 
back  and  forth  from  fresh  to  salt  water? 

5.  Accumulate  data  concerning  the  habitat,  food,  breeding  habits,  distribu- 
tion, economic  importance   (with  the  reasons  therefor)  of  some  of  the  following 
fishes:  salmon,  trout,  white-fish,  sunfish,  muskalonge,  herring,  eel,  cod,  flat-fish, 
mackerel,  shark,  ray,  sturgeon,  gar-pike,  bowfin. 

6.  What  is  known  of  the  habits  of  the  lung-fishes  calculated  to  suggest  how  the 
lung  may  be  of  value  in  preserving  the  life  of  the  animals? 

7.  Migrations  among  fishes. 

8.  Parental  care  among  fishes. 

9.  The  number  of  eggs  produced  by  various  species. 

10.  Study  figures  showing  the  embryology  of  the  salmon  or  other  bony  fish. 

11.  The  blind  fishes  found  in  caves.     What  are  the  principal  facts  concerning 
them,  and  what  explanations  have  been  offered  to  account  for  their  habits  and 
modifications. 

12.  Collect  all  the  data  possible  concerning  the  flat-fishes. 

13.  Examine  all  the  figures  of  fishes  found  in  your  library,  and  make  note  of 
the  chief  points  of  variation  and  the  range  of  these. 


CHAPTER  XXI 

CLASS  II.— AMPHIBIA  (FROGS,  TOADS,  SALAMANDERS) 

394.  The  amphibians  are  especially  interesting  to  the  zoolo- 
gist because  they  begin  life  as  gill-breathers  (tadpoles) ,  and  later 
they  replace  the  gills  by  lungs.     This  is  the  meaning  of  the  name. 
The  fact  that  the  amphibian  in  its  individual  life  passes  from  a 
fish-like  condition  to  the  form  and  habits  of  the  higher  air- 
breathing  vertebrates  is  taken  as  evidence  that  the  higher  ver- 
tebrates have  sprung  from  fish-like  ancestors  through  forms 
similar  to  the  amphibians.     The  change  from  gills  to  lungs  is 
not  equally  striking  in  all  the  members  of  the  group.     The 
transition  from  water  to  air  involves  important  changes  in  the 
problem  of  physical  support,  of  locomotion,  and  of  respiration, 
and  in  consequence,  of  the  organs  performing  these  functions  as 
well  as  correlated  changes  in  the  integument  and  in  the  organs 
of  circulation.     The  amphibia  were  much  more  abundant  in 
earlier  geological  times  than  at  present,  and  attained  huge  size, 
whereas  the  modern  forms,  with  a  very  few  exceptions,  are  small. 
There  are  nearly  two  thousand  living  species.     The  tailless  types 
(frogs  and  toads)  are  much  the  more  numerous,  as  well  as  more 
highly  developed. 

395.  General  Characters. 

1.  Amphibia  are  Vertebrata  which  possess  gills  during  the 
larval  stage  and  usually  lungs  in  the  adult;  in  some  instances 
the  gills  are  retained  throughout  life. 

2.  Paired  appendages  when  present  conform  to  the  general 
vertebrate  type;  i.e.,  limbs  with  digits  (typically  five) ,  instead 
of  fins. 

3.  Exoskeleton  of  scales  and  plates  absent;  skin  glandular. 

4.  Heart  is  three-chambered;  two  auricles  and  one  ventricle. 

5.  A   renal-portal    and   hepatic-portal    circulation    present. 
Red  corpuscles  are  nucleated. 

375 


376  ZOOLOGY 

6.  A  cloaca  occurs,  into  which  the  anus  and  the  ducts  from 
the  excretory  and  genital  organs  open. 

7.  Development  usually  by  a  metamorphosis.     Segmenta- 
tion total  but  unequal. 

396.  Form. — Amphibia  differ  much  as  to  the  shape  of  the 
body.     The  newts  and  salamanders  are  elongated,  slender  and 
eel-like;  the  frogs  and  toads  have  large,  flat  heads,  stout  trunk, 
and  muscular  limbs.     Among  the  former  groups  there  may  be 
as  many  as  two  hundred  and  fifty  body  segments,  in  the  latter 
the  vertebras  behind  the  head  are  reduced  to  ten.     The  neck  is 
usually  inconspicuous,  the  head  being  poorly  movable. 

397.  Appendages. — There  may  be  two  pairs  of  appendages, 
one  pair,  or  none  at  all.     In  most  forms  except  the  Anura  (tail- 
less) the  limbs  are  small  and  weak  as  compared  with  the  body 
(Fig.  191).     The  limbs  have  a  distinct  dorsal  and  ventral  (palmar 
surface,  as  well  as  an  anterior  and  a  posterior  border.     The 
digits  are  enumerated  from  the  anterior  border  which  terminates 
in  the  first,  or  thumb.     In  many  forms  there  is  a  reduction  of 
the  digits  on  the  anterior  appendage  from  five  to  four.     The 
digits  are  almost  universally  destitute  of  claws.     The  feet  are 
often  webbed,  and  in  the  climbing  toads  the  digits  may  end  in 
discs  by  which  they  cling  to  objects. 

398.  The  skin  is  normally  soft,  and  slimy  by  reason  of  a 
glandular  secretion.     It  is  composed  of  two  layers,  epidermis 
and  dermis.     In  the  frog  the  epidermis  is  in  two  layers,  the 
outer  of  which  may  be  shed  at  intervals.     In  toads,  and  other 
forms  frequenting  dry  places,  the  epidermis  may  form  warty 
thickenings.     The  skin  is  often  highly  colored  owing  to  the 
presence  of  pigment  cells  in  the  deeper  layers.     There  are  two 
chief  kinds, — black  and  yellow  pigment  cells.     The  pigment 
cells  are  much  branched,  and  the  pigment  may  either  be  con- 
centrated or  diffused  through  the  cells.     The  different  colors 
the  due  to  different  concentration  and  different  proportions  of 
are  various  pigments.     In  some  cases  the  tones  of  color  may  be 
changed  in  accordance  with  the  surroundings  by  direct  action  of 
light  on  the  pigment  cells,  or  by  the  reflex  nervous  action  of  the 


AMPHIBIA  377 

animal,  resulting  from  impressions  on  the  retina  of  the  eye. 
Besides  light, — heat  and  cold,  moisture,  and  probably  internal 
states  of  the  animal,  may  produce  color  changes.  In  many 
instances  these  colors  are  protective  by  making  the  animal  like 
its  surroundings.  In  the  extinct  Labyrinthodonts  external  pro- 
tective plates  were  developed  in  the  dermis.  Minute  dermal 
scales  are  found  in  some  of  the  lowest  present  forms  ("blind- 
worms"). 

399.  The  Skeleton. — The  points  of  contrast  with  the  skele- 
ton of  fishes  are,  chiefly:  the  presence  of  a  sternum  (formed  in- 
dependently of  the  ribs) ;  the  imperfect  development  of  the  ribs; 
the  typical  limb  skeleton ;  the  union  of  the  pelvic  girdle  with  the 
spinal  column;  the  closer  fusion  of  the  upper  jaw  with  the 
cranium. 

The  vertebrae  of  the  lower  forms  are  biconcave  as  in  fishes, 
in  the  higher  forms  (Anura,  and  higher  Urodela),  concavo-convex. 
The  vertebral  column  usually  consists  of  one  cervical  vertebra; 
a  variable  number  of  thoracic  or  abdominal  vertebrae ;  one  sacral, 
to  which  the  posterior  girdle  is  attached ;  and  a  variable  number 
of  caudal  (one,  in  Anura). 

400.  Respiration. — In  early  larval  stages  the  respiration  is 
effected  wholly  by  means  of  the  skin,  and  even  after  the  develop- 
ment of  special  organs  of  respiration  the  skin  continues  to  serve 
this  function  in  a  greater  or  less  degree.     Most  amphibians  have, 
when  hatched,  external  gills  which  may  be  retained  through 
life  (as  in  Siren,  the  "mud-eel"),  or  may  give  place  to  internal 
gills  covered  by  a  fold  of  skin  (as  in  the  development  of  the  frog). 
Typically,  lungs  replace  both  kinds  of  gills  in  the  adult.     The 
gill  slits  do  not  exceed  three  or  four  pairs  in  number.     Some  of 
the  aquatic  forms  retain  their  gills  when  the  lungs  are  developed, 
each  method  of  respiration  supplementing  the  other.     Those 
which  possess  lungs  alone  in  the  adult  must  of  necessity  undergo 
profound  changes  in  passing  from  the  water-breathing  to  the 
air-breathing  habit.     The  lungs  arise  as  a  ventral  outgrowth 
from  the  esophagus  or  pharynx.     From  the  short  trachea  the 
two  sac-like  lungs  spring.     The  walls  are  in  folds  but  the  sacs 
are  simple.     In  some  salamanders  there  are  neither  gills  nor 


ZOOLOGY 

lungs  in  the  adult,  respiration  taking  place  wholly  through  the 
body  surfaces.  The  frog  breathes  through  its  nostrils.  The 
mouth  cavity  can  be  increased  by  muscular  action,  thus  allowing 
the  entrance  of  air.  The  nasal  openings  are  then  closed  by 
flaps  and  the  air  is  forced  by  muscular  action  into  the  lungs. 

401.  Supplementary  Exercises  for  the  Library. — Find  as  many  different  types 
of  respiration  as  possible  among  the  amphibians,  and  cite  examples.  What  forms 
have  gills  only?  What  evidence  is  there  that  the  environment  has  much  to  do 
with  hastening  or  retarding  the  change  from  gills  to  lungs?  Give  the  natural 
history  of  the  Mexican  axolotl  as  far  as  respiration  is  concerned.  Are  any  Amphibia 
hatched  with  lungs  at  the  outset? 

402.  Circulation. — In  the  gill-breathing  larvae  the  circula- 
tion is  quite  similar  to  that  in  fishes  (§385;  Fig.  181).  When 
the  gills  are  lost  and  lungs  developed,  the  arterial  arches  (Fig. 
169)  which  supply  the  gills  change  their  course,  or  suffer  de- 
struction. This  is  an  interesting  instance  of  the  modification 
of  old  structures  to  meet  new  demands.  Coupled  with  these 
changes  we  find  the  separation  of  the  auricle  into  two  chambers 
—right  and  left.  The  veins  from  the  lungs  empty  into  the  left, 
and  the  systemic  veins  into  the  right  auricle.  While  there  is 
only  one  ventricle  into  which  both  the  pure  blood  from  the  lungs 
and  the  venous  blood  from  the  system  go,  it  is  so  arranged  that 
the  venous  blood  is  chiefly  returned  to  the  lungs  and  the  purest 
blood  goes  to  the  head  and  to  the  systemic  circulation.  The 
venous  circulation  is  modified  in  general  accordance  with  the 
changes  in  the  heart  and  arteries. 

403.  Supplementary  Exercise. — Compare  the  arterial  vessels  in  the  adult 
frog  with  those  in  the  fish  and  the  tadpole  stage  of  the  frog,  and  find  what,  in  the 
opinion  of  various  authors,  is  the  fate  of  each  of  the  arterial  arches.  See  Figs. 
166-169.  What  are  the  most  important  differences  in  the  venous  circulation  in 
fishes  and  in  adult  amphibians? 

404.  Locomotion. — In  the  lower  Amphibia,  in  which  the 
appendages  are  poorly  or  not  at  all  developed,  the  muscles  of 
the  body  show  the  segmental  arrangement  seen  in  fishes,  and 
locomotion  is  effected  by  a  serpentine  or  eel-like  action  of  the 
body.  In  the  higher  forms,  especially  the  Anura,  the  limbs 
are  well  developed ;  and  the  body  muscles  lose  something  of  the 
regularity  of  their  arrangement  and  become  more  as  we  find 
them  in  the  higher  vertebrates.  The  muscles  that  move  the 


AMPHIBIA  379 

limbs  in  relation  to  the  body  come  to  overlie  and  obscure  the 
axial  muscles  proper.  The  Anura  (frogs  and  toads)  are  espe- 
cially adapted  to  leaping  and  swimming  by  the  great  muscular 
development  of  the  hind  legs. 

405.  Exercise. — Are  there  any  special  advantages  in  the  leaping  habit  of 
motion  either  in  the  capture  of  prey  or  in  escape  from,  enemies?  Verify  from  be- 
havior of  toads  and  frogs.  Can  you  find  illustrations  from  other  groups  of  animals ? 

406.  Habits  and  Habitat. — There  are  no  marine  Amphibia. 
Nearly  all  live  in  or  near  the  fresh-water  streams,  swamps,  or 
ponds,  even  in  the  adult  stage.     Some  are  good  climbers  (tree- 
toads);  others  burrow.     The  tailless  forms  (Anura)  are  found 
the  world  over.     The  Urodela  belong  chiefly  to  the  northern 
hemisphere.     All  are  more  abundant  in  warmer  climates.     Their 
food  consists  largely  of  insects,  worms,  and  the  smaller  animals. 
The  larvae  even  of  carnivorous  forms  are  sometimes  vegetable 
feeders.     They  may  live  for  a  long  time  without  food,  and  sur- 
vive the  winter  in  the  colder  latitudes  by  burrowing  deep  into 
the  mud  at  the  bottom  of  their  ponds,  or  otherwise  hibernating. 
During  this  time  the  vital  processes  are  suspended  or  much 
reduced. 

407.  Reproduction   and  Development. — The  common   am- 
phibia lay  rather  large  eggs,  with  a  considerable  amount  of  yolk 
which  results  in  more  or  less  unequal  cleavage  (Fig.   13,  B). 
The  eggs  are  usually  surrounded  by  a  gelatinous  material,  for 
their  protection  and  adhesion,  but  they  have  no  shell.     They 
are  almost  universally  deposited  in  the  water,  where  impreg- 
nation takes  place.     In  some  of  the  Urodela  impregnation  is 
internal.     In  occasional  species  the  young  are  brought  forth  alive. 
Ordinarily  further  development  takes  place  in  the  water  without 
any  attention  from  the  parents  (frogs  and  toads).     In  a  small 
South  American  frog  (Rhinoderma)  the  male  carries  the  fertilized 
eggs  in  his  vocal  sacs  until  hatched ;  in  one  of  the  tree-frogs  from 
South  America  the  female  has  a  pouch  on  the  back  in  which 
the  eggs  are  stored  and  hatched ;  in  the  Surinam  toad  the  eggs  are 
placed  by  the  male  on  the  back  of  the  brooding  female,  where 
they  become  surrounded  by  spongy  tissue.     In  these  pits  they 
hatch  at  once  into  the  adult  form  without  having  external  gills. 


380 


ZOOLOGY 


FIG.  190. 


PlG.    190.     The  metamorphosis  of  the  Frog.      (After  Brehm.)      Numbers  indicate  the  sequence 

Questions  on  the  figures. — How  much  of  the  egg  is  really  ovum?  What  are 
the  changes  which  take  place  in  passing  through  the  various  stages?  In  what 
order  do  the  legs  appear?  How  is  respiration  effected  after  stage  6?  After  stage 
n?  What  is  proven  by  the  collecting  of  the  tadpoles  as  shown  in  3?  How  do 
they  retain  their  position. 


FIG.  191. 


FIG.   IQI.     Tailed  Amphibians.     From  Nicholson,  after  Mi vart.    A,  Siren;  B,  Amphiuma[lridactyla; 

C,  Necturus, 

Questions  on  the  figures. — Compare  these  three  types  and  note  all  the  chief 
differences  of  external  structure.  Compare  also  with  figures  you  may  be  able  to 
find  of  other  Urodela. 


AMPHIBIA 


rJ: 


FIG.   192.     Frog  (Rana).     Photo  from  life  by  J.  W.  Folsom. 

Questions  on  the  figure. — What  is  the  round  object  behind  the  eye?     What 
elements  in  the  resting  position  of  the  frog  put  him  in  readiness  for  a  quick  spnng? 


FIG.   193. 


FIG.   193.     The  Common   Toad    (Bufo  lentiginosus).     Photo   from   life   by    Folsom. 

Questions  on  the  figure. — Compare  with  the  figure  of  the  frog  and  note  points 
of  external  similarity  and  difference.  What  do  you  know  of  the  habits  of  the  toad, 
— as  to  feeding,  egg-laying,  etc.?  Where  do  they  spend  the  winter?  What  of 
their  development? 


382  ZOOLOGY 

This  is  of  course  a  successful  adaptation  (by  eliminating  the 
metamorphosis)  to  a  completely  aerial  habit. 

From  this  group  we  have  beautiful  illustrations  of  unequal 
cleavage  of  the  ovum,  of  which  the  student  should  have  the  op- 
portunity of  seeing  figures  in  more  extended  works.  If  possible 
the  cleaving  eggs  should  be  studied. 

408.  Place  in  Nature. — The  amphibians  are  among  the  oldest 
vertebrates.     They  are  not,  however,  a  very  successful  group  as 
measured  by  numbers  of  species  or  of  individuals.     The  class 
itself  and  such  species  as  the  frog  suggest  the  way  in  which  the 
zoologist    thinks   the   land    animals   arose   from   the   aquatic. 
Among  amphibians  we  have  types  that  are  like  fish,  gill-breathers 
all  through  life;  others  begin  as  gill-breathers,  retain  their  gills 
and  develop  lungs;  still  others  lose  their  gills  when  the  lungs 
are  perfected;  and  some  are  hatched  out  with  lungs,   never 
having  had  functional  gills.     This  makes  a  very  suggestive 
series  of  connecting  links  between  water  and  air-breathing  forms. 

None  of  the  species  has  large  individuals.  The  "giant 
salamander"  of  Japan  is  the  largest,  reaching  a  length  of  3-5 
feet. 

Amphibians  are  practically  uniformly  helpful  to  human  in- 
terests, or  at  least  without  hurt  to  them.  The  toads  destroy 
many  injurious  insects,  and  are  thus  particularly  valuable  about 
gardens.  Toads  are  easily  reared  artificially  and  might  be  made 
to  give  us  even  more  aid  in  controlling  this  difficult  group.  The 
legs  of  some  of  the  larger  species  of  frogs  are  eaten.  The  flesh 
is  white  and  delicate.  Frogs  are  also  used  in  great  numbers  in 
the  zoological  laboratories,  for  dissection  and  experimentation. 
They  have  thus  contributed  no  little  to  our  knowledge  of  animal 
functions,  particularly  the  functions  of  the  nervous  system. 

Profitable  frog  "farms"  are  being  conducted  in  various 
parts  of  the  country  to  meet  the  demands  referred  to  above. 
This  suggests  possible  uses  for  certain  swamp  lands  not  now 
suitable  for  other  purposes.  Edible  frogs  and  turtles  might  be 
encouraged  in  such  regions  instead  of  inedible  types. 

409.  Special  Exercises. — Describe  the  life  history  and  the 
stages  in  the  metamorphosis  of  the  frog.     (Fig.   190.)     What 


AMPHIBIA  383 

larval  organs  disappear?  What  new  organs  are  introduced? 
Compare  other  amphibians  as  to  the  degree  and  facts  of  meta- 
morphosis. 

410.  Classification  of  Amphibia. 

Order  I.  Gymnophiona. — Degenerate  Amphibia  with  neither  legs  nor  tail; 
body  worm-like;  no  gills  nor  gill-slits  in  the  adult;  eyes  more  or  less  degenerate. 
Scales  are  present  in  the  skin.  Represented  by  the  so-called  blind-worms  of 
tropical  countries. 

Order  II.  Urodela. — Amphibia  with  tails  persistent  throughout  life;  body 
elongated;  usually  two  pairs  of  appendages  (sometimes  only  the  anterior  are 
present),  which  may  be  poorly  developed. 

The  principal  suborders  are: 

1.  Perennibranchiata,  in  which  the  gills  persist  throughout  life  (Necturus  or 
water-dog,  Siren  or  mud-eel,  and  certain  blind  forms  found  in  caves). 

2.  Derotremata,  losing  the  gills  in  the  adult  but  retaining  a  spiracular  opening; 
in  the  side  of  the  neck  which  represents  the  gill-slit.     (Examples:  " Congo-snake '* 
of  the  gulf  states,  giant  salamander  of  Japan.) 

3.  Myctodera,  which  lose  all  traces  of  water-breathing.     (Examples:  Newts, 
salamanders,  etc.) 

Order  III.  Anura. — Amphibia  in  which  the  tail  is  absorbed  in  the  adult 
condition,  if  present  in  the  embryo.  Two  pairs  of  appendages,  the  posterior  of 
which  are  well  developed.  Undergo  a  metamorphosis  in  which  the  larvse  usually 
have  the  "tadpole"  form,  with  gills  and  tail  but  without  appendages.  All  traces 
of  gills  lost  in  the  adult.  The  Anura  embrace  the  Bufonidas  or  common  toads, 
the  Ranidae  or  common  frogs,  the  Hylidae  or  tree-toads,  and  other  less  common, 
families.  The  Anura  include  the  great  majority  of  the  species  of  Amphibia. 


CHAPTER  XXII 
CLASS  III.— REPTILIA  (LIZARDS,  CROCODILES,  TORTOISES,  SNAKES) 

LABORATORY  WORK 

411.  Specimens  of  reptiles  are  scarcely  abundant  enough  to 
serve  as  satisfactory  laboratory  types  for  elementary  classes, 
but  instructive  comparisons  may  be  made  by  single  students  or 
by  groups  of  students.  These  results  should  be  reported  to  the 
class. 

Prepare  three  parallel  columns,  one  for  the  lizard,  one  for 
the  snake,  and  one  for  the  turtle.  Select  a  specimen  of  each 
and  compare  them  with  regard  to  their  haunts;  habits;  food; 
general  form  of  body;  appendages,  number,  position,  joints, 
digits;  covering;  manner  of  locomotion. 

412.  Special  Topics  for  Investigation  in  the  Laboratory  and  Field. 

1.  Are  reptiles  warm  or  cold  blooded?     Your  evidences? 

2.  What  are  the  differences  between  the  scales  of  snakes  and  of  fishes? 

3.  In  what  various  ways  is  the  tail  of  reptiles  used  as  an  organ?     How  is  the 
tail  to  be  distinguished  from  the  rest  of  the  body  ? 

4.  What  special  senses  do  reptiles  possess?     What  are  your  evidences?     What 
peculiarities  have  the  organs  of  sense? 

5.  What  peculiarities  do  the  internal  organs  of  the  snake  have  which  seem  to 
be  correlated  with  the  slender,  elongate  form  of  the  animal  ? 

6.  What  species  of  snakes,  turtles,  and  lizards  are  found  in  your  locality? 
Report  on  the  special  habits  of  each  species  in  so  far  as  you  can  determine  them 
by  observation.     Supplement  by  reference  to  authorities. 

DESCRIPTIVE  TEXT 

413.  TheReptilia  differ  from  the  vertebrates  we  have  hith- 
erto studied  in  the  fact  that  at  no  period  of  life  do  they  possess 
gills.  They  agree  with  the  lower  forms  in  being  cold-blooded 
and  in  the  incomplete  separation  of  the  heart  into  right  and  left 
compartments  (except  in  the  crocodiles) .  They  are,  in  addition 
to  their  air-breathing  habit,  similar  to  the  birds  and  mammals 
in  possessing  the  protective  embryonic  membranes  known  as 

384 


REPTILIA  385 

the  amnion  and  allantois  (see  §423),  the  latter  of  which  is  im- 
portant in  embryonic  respiration,  that  is,  before  hatching  or 
birth.  The  group  reached  its  culmination  in  numbers,  variety 
and  size  in  the  Mesozoic  age.  So  true  is  this  that  the  Mesozoic 
is  called  the  "Age  of  Reptiles."  Those  we  have  at  present  are 
to  be  looked  upon  as  specialized  and,  in  some  instances  (snakes) 
perhaps,  degenerate  remnants  of  the  first  vertebrate  class  wholly 
to  give  up  breathing  by  means  of  gills.  In  the  Mesozoic  era 
there  were  immense  swimming,  fish-like  forms  (ichthyosaurs  and 
plesiosaurs)  which  ruled  the  seas;  powerful  terrestrial  dinosaurs, 
often  walking  on  their  hind  legs,  and  including  the  largest  land 
animals  known  to  have  lived;  and  others,  with  membranous 
wings  like  the  bat,  the  first  vertebrates  to  learn  the  art  of  flying 
(Fig.  196).  With  the  exception  of  a  few  marine  turtles,  the 
boas  and  pythons,  and  the  alligators  and  crocodiles,  the  living 
species  are  for  the  most  part  small  animals. 

414.  General  Characteristics. 

1.  Reptilia  are  usually  covered  with  scales  or  plates  derived 
from  the  dermis  (bony),  or  the  epidermis  (horny),  or  from  both. 

2.  The  (3-5)  digits  when  present  are  provided  with  claws. 

3.  The  vertebrae  are  concavo-convex,  usually  concave  in 
front  and  convex  behind. 

4.  The  heart  is  three  chambered; — that  is,  the  auricles  are 
completely  separated,  but  the  ventricles  are  only  partially  so 
except  in  the  Crocodilia. 

5.  There  are  two  aortic  arches,  a  right  and  a  left,  in  the  adult. 

6.  Gills  do  not  occur  at  any  period. 

7.  Reptiles  are  chiefly  oviparous;  the  eggs  are  large,  well 
supplied  with  yolk,  arid  protected  by  a  leathery  shell. 

8.  The  embryonic  membranes, — amnion  and  allantois — first 
make  their  appearance  in  this  group. 

415.  The  Reptiles  are  very  diverse  in  form.     Perhaps  the 
lizards  may  be  taken  as  typical,  with  cylindrical  body,  more  or 
less  distinct  head  and  neck,  distinct  tail,  and  usually  two  pairs 
of  appendages,  each  possessing  five  digits  armed  with  claws. 
They  are  mostly  small  animals,  though  one  species  is  known  to 
attain  a  length  of  five  feet.     The  crocodiles  and  alligators  are 

25 


386  ZOOLOGY 

similar  in  shape  but  much  larger.  The  turtles  and  snakes  are 
most  widely  different  from  the  type  and  must  be  regarded  as 
much  specialized,  or  even  degenerate,  forms.  The  turtles  have 
sought  protection  by  means  of  a  bony  box,  and  are  ill  adapted 
for  motion  either  on  land  or  water.  Snakes,  on  the  other  hand, 
elongated  and  devoid  of  appendages,  are  among  the  most  rapid 
and  graceful  of  animals  in  their  motions.  The  long  tapering 
body  is  a  successful  prehensile  organ.  Some  of  the  lizards  agree 
with  the  snakes  in  lacking  legs. 

416.  Covering. — The  external  covering  in  reptiles  is  in  the 
form  of  scales  or  plates  formed  by  the  epidermis,  or  the  dermis, 
or  both.     That  deposited  by  the  epidermis  is  horny  and  that 
by  the  dermis,  bony.     In  snakes  and  many  lizards  the  scales 
are  epidermal  and  may  be  periodically  shed  and  renewed.     The 
scales  usually  differ  in  shape  and  size  in  different  parts  of  the 
body.     In  turtles  and  their  allies  the  horny  constituent,  which 
is  illustrated  by  the  "tortoise  shell"  of  commerce,  is  in  the  form 
of  plates  and  is  reinforced  by  bony  dermal  plates  beneath. 
The  latter  do  not,  in  the  a'dult  at  least,  correspond  in  number  and 
size  with  the  former,  but  are  closely  associated  with  the  bones 
of  the  internal  skeleton.     In  crocodiles  the  dermal  scales  cor- 
respond in  general  with  the  epidermal. 

The  members  of  the  group  are  on  the  whole  well  protected 
by  these  external  growths.  In  many  instances,  as  in  certain  of 
the  lizards,  prominent  projections  are  formed  upon  the  body 
covering,  giving  a  striking  appearance  to  the  animals. 

417.  Internal  Skeleton. — The  vertebral  column,  except  in 
the  snakes  and  snake-like  lizards,  shows  the  customary  regions 
(see  §349).     In  the  limbless  forms  only  two  regions  are  recog- 
nized,— the  pre-caudal  which  bear  the  ribs,  and  the  caudal  or 
tail  vertebrae.     The  vertebrae  are  usually  concave  in  front  and 
convex  behind,  thus  making  a  kind  of  ball-and-socket  joint. 
In  snakes  the  number  of  vertebrae  is  very  large.     No  sternum 
occurs  in  turtles  and  snakes.     When  present,  as  in  lizards  and 
crocodiles,  it  is  formed  in  connection  with  the  ventral  end  of  the 
ribs. 

'      The  skull  articulates  with  the  first  vertebra  by  one  surface 


REPTILIA  387 

(condyle)  instead  of  two  as  in  mammals.  The  lower  jaw  articu- 
lates indirectly,  by  means  of  the  quadrate  bone,  with  the  skull. 
This  gives  a  very  movable  jaw  and  in  the  snakes  especially, 
increases  the  caliber  of  the  throat  (Fig.  194).  The  cranium  is 
more  compactly  fused  and  completely  ossified  than  among  the 
Amphibia.  The  ulna  and  radius  and  the  tibia  and  fibula  are  not 
fused  as  in  the  frog.  Rudiments  of  the  pelvic  girdles  are  found 
in  some  snakes,  although  the  limbs  are  wanting. 


FIG.  194.  Skull  of  Rattlesnake  (Crotalus  durissus).  From  Nicholson,  after  Huxley,  or, 
articular  portion  of  lower  jaw;  de,  dentary  portion;  bo,  basi-occipital;  mx,  maxilla,  bearing  poison 
fang;  no,  nasal;  pi,  palatine,  the  front  end  being  represented  by  a  dotted  line  as  though  seen  through 
the  maxilla;  pmx,  premaxilla;  po,  post  frontal;  pr,  prefrontal;  pt,  pterygoid;  qu,  quadrate;  sq,  squa- 
mosal;  tr,  transverse  bone. 

Questions  on  the  figure. — Which  bones  bear  teeth?  Which  are  cranial  and 
which  facial  bones?  What  bones  do  you  find  common  to  the  snake  and  the  fish 
(Fig.  1 80)?  How  do  they  differ  in  the  two  forms?  What  is  the  function  of  the 
quadrate?  How  does  it  differ  in  the  different  groups  of  Vertebrates? 

418.  Respiration. — Functional  gills  never  occur,  though  gill- 
slits  are  partly  developed  in  the  embryo  only  to  close  again 
before  hatching.  The  trachea  is  elongated  and  is  supported  by 
cartilaginous  rings  as  in  the  higher  forms.  It  divides  into  two 
bronchi,  each  of  which  passes  to  a  spindle-shaped  sac — the  lung 
—which  is  much  simpler  in  its  lobings  than  those  of  birds  and 
mammals.  In  the  snakes  one  lung  (the  left)  is  much  reduced 


388 


ZOOLOGY 


or  even  altogether  aborted.  This  is  an  adaptation  to  the  narrow 
elongated  body  cavity.  The  ribs  when  present  and  the  muscles 
acting  on  them  are  the  prime  agents  in  breathing. 

419.  Circulation. — In  reptiles  the  right  and  left  auricles  are 
entirely  distinct  but,  with  the  exception  of  the  Crocodilia,  the 
ventricles  are  only  partially  so.  The  septum  in  the  ventricle  is 
perforated.  Yet  in  those  forms  in  which  the  pure  blood  of  the 
left  auricle  and  the  impure  blood  of  the  right  partially  mingle 
in  the  ventricle,  the  arrangement  is  such  that  the  purest  blood 

FIG.  195. 


pr.  z, 


pt.  z 


t.  p.— - 


FIG.  195.  Vertebrae  of  a  Reptile  (after  Huxley).  A,  anterior  view;  B,  posterior  view  of  the 
vertebra  in  front  of  A.  The  surface  of  A  fits  against  the  surface  of  B.  c,  centrum,  which  is  convex 
in  B,  fitting  into  a  concavity  in  A;  n.s.,  neural  spine;  pr.z.,  pre-zygapophyses,  or  anterior  articular 
facets,  which  fit  against  pt.z.,  post-zygapophyses;  t.p.,  transverse  processes;  z.s.,  a  wedge-like 
articular  face  on  the  neural  arch  designed  to  fit  into  z.c.,  a  depression  on  the  posterior  face  of  the 
neural  arch  of  the  vertebra  in  front  (5). 

Questions  on  the  figures. — Try  to  form  a  clear  picture  of  the  relations  of  the 
articulating  surfaces  of  the  vertebrae  and  indicate  the  possible  advantages  of  the 
arrangements.  Where  is  the  neural  cavity?  Where  do  the  ribs  articulate? 
What  is  the  gain  in  muscular  attachments  from  the  numerous  bony  outgrowths 
on  the  vertebrae? 

goes  to  the  brain  and  the  least  pure  to  the  lungs  (see  Fig.  170). 
Two  aortic  arches  unite,  giving  rise  to  the  dorsal  aorta.  In  the 
reptiles  and  higher  groups  of  vertebrates  the  renal-portal  circu- 
lation (see  Fig.  181,  r.p.)  ceases  to  be  of  much  importance,  but 
the  hepatic  portal  is  increasingly  important.  The  red  corpuscles 
are  elliptical  and  possess  nuclei. 

420.  Nervous    System   and    Special    Sense    Organs. — The 

brain  is  not  large  in  the  reptiles,  but  is  rather  more  highly  de- 


REPTILIA  389 

veloped  than  in  the  Amphibia.  This  is  especially  true  of  the 
cerebral  hemispheres.  The  usual  senses  are  represented.  The 
rather  large  eyes  are  provided  with  movable  eyelids  except 
among  the  snakes,  in  which  a  permanent  transparent  membrane 
covers  the  eye.  In  some  reptiles  (lizards)  there  is  a  remnant 
of  a  median  eye  which  is  hopelessly  degenerate  in  the  adult. 
It  is  in  connection  with  the  pineal  body  in  the  second  division 
of  the  brain.  Hearing  varies.  It  is  rather  keen  in  turtles; 
apparently  less  so  in  snakes.  Smell  is  well  developed.  Touch 
is  interfered  with  by  the  scales  and  plates,  but  is  represented 
in  the  skin  over  the  body. 

421.  Habits. — The  reptiles  are  best  represented  in  the 
tropical  regions.  The  larger  types,  as  the  crocodiles,  python, 
boa  are  almost  confined  to  the  warm  zones,  especially  of  South 
America,  Africa  and  Asia.  Numerous  smaller  representatives 
of  the  lizards,  snakes,  and  turtles  are  found  in  temperate  lati- 
tudes. These  usually  undergo  a  period  of  hibernation  during 
the  cold  season.  This  habit  of  hibernating  and  seeking  warmer 
climates  seems  related  to  the  cold-blooded  condition.  The  heat- 
producing  qualities  of  the  animals  are  not  equal  to  the  task  of 
maintaining  activity  during  extreme  cold.  The  variation  of 
temperature  is  of  course  a  more  serious  problem  to  terrestrial 
animals  than  to  aquatic  types.  Although  air-breathers,  very 
many  of  the  group  are  aquatic,  as  the  turtles,  crocodiles,  and 
many  snakes.  The  lizards  are  almost  without  exception  terres- 
trial. Nearly  all  prey  on  other  animals;  the  smaller  on  worms, 
insects,  and  eggs  of  various  kinds,  and  the  larger  on  birds,  fish, 
amphibia,  and  mammals.  The  land  tortoises  are  vegetable 
feeders. 

Reptiles  are  rather  sluggish  animals.  They  like  to  bask  in 
the  sun.  They  are  capable  often  of  very  rapid  motion;  but  it 
is  spasmodic. 

Reptiles,  especially  the  snakes,  have  a  bad  reputation,  yet 
there  is  no  doubt  that  their  dangerous  qualities  are  much  ex- 
aggerated in  popular  opinion.  The  lizards  are  almost  wholly 
non- venomous  and  the  majority  of  the  common  snakes  of  this 
country  are  also  harmless.  The  principal  dangerous  snakes 


390 


ZOOLOGY 


are  the  cobra  of  the  East  Indies  where  nearly  25,000  deaths 
were  caused  by  serpents  in  1899 ;  the  vipers  of  Europe;  the  rattle- 
snakes, water-moccasin,  and  copperhead  of  our  own  country. 
There  are  about  twenty  poisonous  species  in  this  country  of 
which  fifteen  species  are  rattle-snakes.  The  venom  serves  the 
snake  both  as  a  means  of  defense  and  of  paralyzing  its  prey. 
Many  forms  which  are  not  poisonous  assume  bodily  attitudes 
similar  to  those  of  the  poisonous  species.  This  is  known  as 
mimicry,  and  is  a  means  of  protection.  The  dangerous  species 
are  being  rapidly  exterminated  by  man. 

FIG.  196. 


PIG.   196.     Rhamphorhynchus  merensteri, — a  restoration  of  an  extinct  flying  Reptile.     From  the 
Cambridge  Natural  History,  after  Geikie. 

Questions  on  the  figure. — In  what  respects  does  a  form  like  this  differ  in 
external  appearance  from  a  bird  ?  From  a  bat  ?  What  skeletal  structures  would 
a  palaeontologist  need  to  find  in  order  to  believe  that  an  extinct  form  had  the  power 
of  flight? 

422.  Special  Exercises. — Find  data  concerning  hibernation  in  reptiles  and  other 
vertebrates:  its  object  and  advantages;  preparation;  place;  degree  in  which  vitality 
is  suspended  during  the  process,  etc. 

Describe  the  poison  apparatus  of  venomous  snakes.  What  is  the  homology 
of  the  fang?  Of  the  gland? 

How  do  different  snakes  capture  their  prey?     How  prepare  it  for  swallowing? 

Describe  and  attempt  to  explain  the  motion  of  snakes  from  actual  observation : 
in  water;  on  land. 

423.  Reproduction  and  Development. — The  ova  escape  from 
the  two  ovaries  into  the  body  cavity.  As  in  the  Amphibia  the 
inner  end  of  the  oviduct  opens  well  forward  in  the  body  cavity. 
The  ova  enter  the  oviducts,  and  during  the  descent  are  fertilized. 
After  fertilization  the  glands  in  the  walls  of  the  oviducts  add 
albumen  and  shell  structures,  as  in  the  birds.  The  eggs  require 


REPTILIA  391 

a  period  of  incubation  which  usually  occurs  outside  the  body; 
though  some  lizards  and  snakes  retain  the  eggs  in  a  special  por- 
tion of  the  oviduct  until  the  embryo  is  hatched,  thus  bringing 
forth  their  young  alive.  Many  forms  deposit  their  eggs  in  the 
warm  sand  or  earth  or  in  decaying  rubbish  heaps,  where  the 
abundant  heat  is  favorable  for  the  developing  young. 

Much  yolk  is  present  in  the  egg  and  segmentation  is  partial, 
being  confined  to  a  disc.  The  germinal  layers  and  the  im- 
portant organs  develop  about  the  axis  of  this  disc,  the  outer 
margins  of  which  spread  over  the  whole  yolk  in  the  form  of  a 
sac  designed  to  nourish  the  embryo.  The  details  of  the  growth 
are  entirely  too  complicated  for  statement  here.  Two  important 
embryonic  membranes — the  amnion  and  allantois — appear  for 
the  first  time  (see  also  §440).  The  amnion  consists  of  folds 
of  the  blast odermic  disc  which  arise,  surrounding  the  embryo 
at  its  margin.  These  folds  grow  dorsally  over  the  embryo  and 
ultimately  fuse  to  enclose  a  space*which  becomes  filled  with 
fluid.  The  amnion  folds  include  both  ectoderm  and  mesoderm. 
It  is  protective  in  function  (Fig.  208,  am).  The  cavity  between 
the  two  layers  of  the  amnion  is  an  outgrowth  of  the  ccelom. 
The  allantois  arises  as  a  fold  from  the  posterior  portion  of  the 
digestive  tract,  and  is  made  up  of  entoderm  and  mesoderm. 
It  finally  surrounds  not  merely  the  embryo  but  the  yolk  on  the 
ventral  side,  and  being  well  supplied  with  blood  vessels  is  most 
important  in  supplying  the  embryo  with  oxygen.  In  this  and 
in  other  features  the  reptiles  show  a  close  kinship  with  the  birds. 

424.  Place  in  Nature. — Reptiles  live  upon  both  plant  and 
animal  food.  They  devour  worms,  insects,  fish,  amphibians, 
birds,  small  mammals,  and  eggs.  Not  many  animals  depend 
on  reptiles  for  food.  They  are  more  immune  from  attack  than 
most  animals.  Some  of  the  predaceous  birds  eat  lizards  and 
snakes. 

The  group  does  not  aid  man  at  many  points.  Certain  large 
lizards,  and  the  tortoises  and  turtles  furnish  food.  The  soft- 
shelled  turtle,  the  green-turtle,  and  the  diamond-back  terrapin 
are  the  most  prized. 

Skins  of  crocodiles  and  of  some  snakes,  and  the  horny  tor- 


392 


ZOOLOGY 


toise-shell  are  used  in  the  manufacture  of  bags,  ornaments,  and 
the  like. 

Probably  the  chief  value  of  the  group  to  man  is  in  the  de- 
struction of  noxious  insects  by  lizards  and  rodents  by  snakes. 

425.  Classification  of  Reptiles. 

Order  I.  Chelonia  (Turtles  and  Tortoises}. — The  Chelonia  are  reptiles  with 
short,  flattened  or  dome-shaped  bodies  enclosed  in  a  case  formed  by  a  dorsal  shield 
(carapace)  and  a  ventral  (plastron).  In  some  the  carapace  and  plastron  make  a 
rather  tight-box  practically  covering  the  animal.  In  others  they  are  smaller  and 
the  edges  are  further  apart.  In  these  the  appendages  may  protrude  more,  and 
the  freedom  of  motion  is  much  greater.  The  jaws  are  covered  with  a  horny  case 

FIG.  197. 


FIG.  197. 


Common  Box  Tortoise   (Cistudo  Carolina). 
Shufeldt. 


Photographed  from  life  by   Dr.   R 


and  are  destitute  of  teeth.  The  quadrate  bone  is  firmly  fused  to  the  cranium. 
The  sternum  is  absent.  Turtles  seem  rather  more  common  in  the  northern  hemi- 
sphere. The  largest  species  are  marine  and  may  attain  a  weight  of  half  a  ton. 
Some  live  in  fresh  water  and  others  on  land.  The  flesh  of  some  species  is  much 
prized  for  food.  The  green- turtle  of  the  Atlantic  coast  is  one  of  the  choicest,  its 
flesh  being  much  used  for  soups.  The  large  hawkbill-turtle  of  the  tropical  seas 
furnishes  "tortoise-shell,"  used  in  combs  and  other  ornaments.  The  shells  of 
the  leather-back  and  other  "soft-shelled "  turtles  are  not  completely  ossified.  The 
"snappers"  are  ferocious  animals,  the  big  snapper  of  the  Southern  states  being 
particularly  vicious. 

Order  II.  Lacertilia  (Lizards). — Reptiles  in  which  the  body  is  usually  covered 
with  small  scales.  Two  pairs  of  limbs  are  ordinarily  present;  but  either  or  both 
may  be  wanting.  The  quadrate  bone  is  somewhat  movable.  The  teeth  are  not 
in  sockets  of  the  jaw.  Sternum  present.  The  cloacal  opening  is  transverse. 


REPTILIA 


393 


The  Lacertilia  include,  beside  the  types  commonly  known  as  lizards,  the 
chameleons,  horned-toads,  and  the  glass  snake — a  legless  lizard.  They  subsist 
largely  on  insects  and  the  eggs  of  other  animals.  Only  one  species  is  known  to  be 
poisonous — the  "Gila  monster"  of  New  Mexico  and  southward.  The  glass  snake 
possesses  in  a  high  degree  a  power  more  or  less  common  among  lizards — of  breaking 
loose  from  the  tail  when  struck,  or  held  by  that  organ.  In  some  species,  at  least, 
a  new  tail  may  be  regenerated.  Most  lizards  are  terrestrial,  though  a  few  are 

FIG.  198. 


FIG.  198. 


Swift  Lizard   (Sceloporus  undulatus).     Adult. 
Shufeldt. 


Photographed  from  life  by  Dr.  R.  W. 


aquatic.     They  run,  burrow,  and  climb.     One  species  (Fig.  199)  has  a  membrane 
from  the  sides,  supported  by  ribs,  that  serves  as  a  parachute. 

Order  III.  Ophidia  (Snakes'). — Reptiles  with  elongated  bodies  covered  by 
fold-like  epidermal  scales  which  may  be  shed  as  a  single  "cast."  Limbs  are 
wholly  wanting.  The  mouth  is  capable  of  great  extension  on  account  of  the  great 
movability  of  the  quadrate  and  other  bones.  Teeth  are  numerous  and  fused  (not 
in  sockets)  to  the  bones  bearing  them.  Sternum  wanting.  There  are  no  movable 


394 


ZOOLOGY 


FIG.  199. 


FIG.   199.     Flying  Lizard  (Draco  volitans).     From  Nicholson. 


FIG.  200. 


FIG.  200.      Dorsal  view  of  a  "Gila  Monster"  (Heloderma  suspectum}.     Photographed  by  Dr.  R.  W. 

Shufeldt. 


REPTILIA 
FIG.  201. 


395 


FIG.   201.     Common   Garter  Snake   (Eutcenia  sirtalis).     Photographed  from  life  by   Dr.  R.   W. 

Shufeldt. 


PlG.    202. 


FIG.  202.     Blotched  King  Snake  (Lampropeltis  rhombomaculatus).     Photographed  from  life  by  Dr. 

R.  W.  Shufeldt. 

Questions  on  the  figure.— What  structural  adaptations  have  the'  snakes  which 
tend  to  take  the  place  of  appendages?  Illustrate  your  conclusions  by  citing  ex- 
amples. 


3Q6  ZOOLOGY 

eyelids.  The  tongue  is  protrusible,  and  is  doubtless  much  used  as  an  organ  of 
touch,  and  possibly  hearing. 

Snakes  are,  like  lizards,  partial  to  warm  climates,  but  are  also  found  in  tem- 
perate latitudes.  Most  are  terrestrial,  but  some  take  to  water  readily;  and  there 
are  some  which  never  come  to  land.  These  bring  forth  their  young  alive.  Many 
snakes  are  beautifully  and  characteristically  colored.  In  some  instances  the 
coloration  is  deemed  to  be  protective. 

Order  IV.  Crocodilia  (Crocodiles,  Alligators,  etc.}. — Fresh-water  reptiles 
with  elongated  bodies  bearing  two  pairs  of  well-developed  appendages.  The  skin  is 
armed  with  dermal  bony  scales  or  scutes  covered  by  epidermal  scales.  Teeth 
occur  in  sockets.  The  quadrate  is  immovable  and  the  sternum  is  present.  The 
adult  heart  is  completely  divided  into  right  and  left  halves.  The  cloaca  opens  by 

FIG.  203. 


FlG.  203.     Head  of  the  American  Alligator  (Alligator  Mtssissippiensis).     From  Eckstein. 

a  longitudinal  slit.  Here  are  included  the  gavial  of  the  Ganges,  the  crocodiles 
of  the  Nile  and  of  tropical  America,  and  the  alligator  of  America.  They  are  some- 
what sluggish  animals,  but  when  hungry  will  attack  with  success  the  larger  mam- 
mals or  man.  They  may  attain  a  length  of  twenty  feet  or  more.  Crocodilia 
are  chiefly  aquatic,  though  they  rest  on  the  shore,  and  deposit  their  eggs  in  the 
sand  where  they  hatch. 

There  are  numerous  orders  of  extinct  reptiles  which  show  close  relationship 
with  the  fishes,  amphibians,  and  birds  of  early  geological  times.  This  is  merely 
another  way  of  saying  that  the  early  Reptilia  and  the  other  vertebrates  were  much 
more  generalized  in  their  characteristics  and  less  differentiated  than  those  of  the 
present. 

426.  Supplementary  Topics  for  Investigation. 

1 .  Have  the  venomous  snakes  any  characteristic  appearance  ? 

2.  Report  on  the  habits  of  the  rattle-snake.     Whence  the 
structure  giving  rise  to  the  name  ?     The  nature  of  the  fang  and 
the  poison  gland. 

3.  What  is  the  degree  of  activity  and  strength  of  the  reptiles 
and  cold-blooded  animals,  as  compared  with  the  warm-blooded  ? 


REPTILIA  397 

4.  The  characteristics  of  the  principal  groups  of  geological 
reptiles. 

5.  The  meaning  of  hibernation.     Illustrations  of  its  method 
among  the  reptiles. 

6.  The  various  methods  of  capturing  prey  among  reptiles. 

7.  The  facts  concerning  incubation  and  care  of  young  among 
reptiles. 

8.  The   development   of   the   amnion   and     allantois.     See 
figures  in  reference  texts. 


CHAPTER  XXIII 
CLASS  IV.— AVES  (BIRDS) 

427.  Laboratory  and  Field  Studies. — Each  student  or  group 
of  students  should  be  encouraged  to  select  one  or  more  species 
of  birds  and  to  study  their  habits  and  external  structure  in  the 
light  of  the  following  outline. 

I.  Habits  and  Activities. 

Haunts  and  feeding  habits.  What  are  its  favorite  foods? 
How  determined  ?  What  emergency  foods  will  it  use  ? 

Social  habits ;  solitary  or  gregarious  ? 

Mating  habits;  monogamous  or  polygamous?  Degree  of 
sexual  dimorphism? 

Determine  and  describe  its  powers  of  song.  Are  they  equally 
developed  in  all  individuals  of  the  species  ?  Can  you  cite 
any  evidence  that  the  power  is  of  any  use  to  the  animals  ? 

Nesting  habits;  number  of  eggs,  their  size  and  other  char- 
acters. Is  their  color  of  any  conceivable  use?  Which 
sex  incubates  the  eggs  ?  Condition  of  the  young  at  hatch- 
ing and  the  care  given  by  the  parents  to  the  young. 

Migrations.  Are  there  any  evidences  of  winter  (or  other 
important)  migrations?  If  not,  how  is  the  winter  spent? 
If  so,  at  what  time  does  it  occur  ?  When  does  the  species 
return  ?  Any  other  known  facts. 

Power  and  peculiarities  of  flight.  Other  modes  of  locomo- 
tion ?  Is  the  power  of  perching  well  developed  ? 

What  are  its  relations  to  the  other  animals  of  the  locality  ? 
Has  it  any  enemies  ?     Is  it  hostile  to  any  species  of  animals  ? 

What  is  its  abundance  or  scarcity  in  your  locality?  Can 
you  assign  any  explanation  of  the  facts  observed  ? 

II.  General  External  Appearance. 

Regions  of  the  body :  head,  neck,  trunk,  limbs. 

Head:  beak,  mouth,  nares,  eyes  (how  many  lids?),  ears. 
Neck:  length,  natural  position,  flexibility,  etc. 
Wings:  arm,  forearm,  hand. 

398 


AVES  399 

Legs :  thigh,  shank,  foot.     Where  is  the  heel  ?     Evidences  ? 
Note  further  arrangement  and  number  of  digits ;  form  of 
the  claws;  covering  of  the  tarso-metatarsus. 
Covering  of  the  body.     Compare  the  color  of  all  visible  parts. 
Select  feathers  from  various  parts  of  the  body;  study  as  a 
type  one  of  the  large  wing  feathers,  noting :  shaft  (quill 
and  rachis),  vane  (barbs  and  barbules). 
Compare  the  other  feathers  selected  with  this  one.     What 

would  you  suggest  as  the  prime  function  of  each  kind  ? 
Arrangement  of  the  feathers. 

On   wings:   remiges    (large),    primary    (on    hand)    and 

secondary  (on  forearm) ;  coverts. 

On  tail:  rectrices,  number  and  arrangement;  coverts. 
On  body  (dip  in  hot  water  and  pluck):  note  the  pits 
which  have  borne  the  feathers;  arrangement  of  these. 
Are  they  uniformly  distributed  over  the  entire  body  ? 
Sketch  the  plucked  bird,  studying  more  carefully  the  regions 
already  noted.     Locate 
Openings:  mouth,  nares,  ears,  cloaca. 
Queries : 

Is  there  any  connection  between  the  closing  of  the  toes 
and  the  flexing  of  the  leg  ?  Has  this  any  use  to  the  ani- 
mal? 

Which  digits  are  represented?  Are  they  equally  devel- 
oped? 

Which  digit  is  turned  backward  ?     How  is  this  determined  ? 
Is  there  a  tongue?     Are  there  teeth?     Do  the  nostrils 

communicate  with  the  mouth  ? 

What  do  you  consider  the  function  of  the  nictitating  mem- 
brane ?    Are  the  eyes  movable  ?     Do  they  view  the  same  field  ? 
Do  the  two  together  cover  the  entire  field  of  view  ? 
How  much  external  ear  is  present  ? 
Are  the  scales  homologous  with  feathers  ?     (See  reference 

texts.) 
III.   Internal   Structures.     (The    pigeon    is    a    good    type    for 

anatomical  study.) 

The  student  knows  what  internal  organs  and  systems  of 
organs  to  expect  in  vertebrates,  and  may  well  be  required 


400  ZOOLOGY 

to  block  out  an  outline  of  work.  What  are  the  principal 
sets  of  organs  to  be  expected?  How  do  these  compare 
with  the  corresponding  structures  in  the  fish  or  frog  ?  Do 
you  find  any  structures  not  found  in  the  lower  types.  Un- 
certainties may  be  settled  by  reference  to  more  extended 
texts. 

DESCRIPTIVE  TEXT 

428.  Birds  must  be  looked  upon  as  sharing  with  mammals 
the  first  place  in  the  animal  kingdom.     Even  the  mammals  as 
a  class  are  not  so  highly  specialized  in  structure  and  in  habits 
as  the  birds.     Their  most  striking  features  of  specialization  are 
connected  with  the  demands  of  aerial  life  which  they  have  so 
successfully   met.     They   share   with   the   insects, — the   most 
specialized  of  the  invertebrate  phyla, — the  most  perfect  de- 
velopment of  the  power  of  flight  found  among  animals.     It 
follows  from  their  high  degree  of  specialization  that  they  are 
among  the  most  easily  recognized  of  the  vertebrates.     The 
earliest  geological  traces  of  birds  show  that  they  are  closely 
linked  with  reptiles  in  their  origin,  and  the  modern  birds  pre- 
serve many  interesting  likenesses  to  the  reptiles.     Some  of  these 
are  seen  in  the  scales  on  the  shank  and  feet  of  birds ;  in  the  habit 
of  laying  large,  well-nourished  eggs  which  hatch  outside  the 
body;  in  the  structure  of  the  egg  and  its  mode  of  cleavage;  the 
peculiarities  of  the  ankle  joint;  in  the  presence  of  a  cloaca. 

429.  General  Characteristics  of  Birds. 

1.  The  Aves  are  vertebrates  in  which  the  epidermic  out- 
growths usually  take  the  form  of  feathers  (or  scales  in  special 
regions,  as  the  feet). 

2.  The  anterior  appendages  in  the  majority  of  forms  are 
modified  for  flight.     Associated  with  this  is  a  large  development 
of  the  pectoral  muscles  and  the  bones  to  which  they  are  attached. 

3.  A  single  occipital  condyle. 

4.  The  heart  is  completely  four-chambered;  only  one  (sys- 
temic) aortic  arch  which  turns  to  the  right;  red  corpuscles  oval 
and   nucleated.     High   bodily   temperature,    100°   to    110°   F. 
Renal  portal  circulation  almost  wanting. 


AVES  401 

5.  Some  of  the  bronchial  tubes  terminate  in  air  spaces  (not 
true  lung  tissue)  located  in  various  parts  of  the  body.     These 
communicate  with  air  cavities  in  some  of  the  bones. 

6.  The  parts  of  the  skeleton  are  much  fused.     There  are  no 
teeth,  the  jaw  being  sheathed  by  a  horny  product  of  the  epi- 
dermis (beak). 

7.  The  right  ovary  and  oviduct  are  aborted  or  rudimentary. 

8.  All  are  oviparous;  yolk  abundant;  segmentation  discoidal; 
amnion  and  allantois  present. 

430.  Form. — The  birds,  like  many  of  the  extinct  reptiles, 
are  bipeds.  The  axis  of  the  more  or  less  stout  body  makes  an 
angle  of  varying  size  with  the  axis  of  the  legs,  that  is,  the  vertical. 
The  sacrum  and  the  soft  parts  of  the  body  project  behind  this 
point  of  union  in  such  a  way  as  to  balance  the  anterior  parts. 
The  anterior  appendages  are  not  always  well  developed  but  are 
much  anterior  to  and  above  the  centre  of  gravity.  This  results 
in  a  more  stable  position  of  the  body  in  flight.  The  posterior 
appendages  are  relatively  long,  sometimes  extraordinarily  so. 
In  all  cases  there  is  an  interesting  correlation  between  the  length 
of  the  neck  and  that  of  the  legs.  The  wading  birds  are  especially 
endowed  in  these  particulars.  The  posterior  appendages  usually 
have  four  digits.  These  may  all  be  directed  forward  as  in  some 
swifts,  or  much  more  commonly  the  great  toe  (number  i)  is 
directed  backward ;  in  some  species  two  are  turned  backward  and 
two  forward.  In  swimming  birds  a  web  is  present  which 
stretches  from  toe  to  toe.  The  special  form  and  arrangement 
of  the  web  differ  in  different  species.  The  digits  end  in  claws 
which  vary  greatly  in  accordance  with  the  habits  of  the  possessor. 
The  anterior  appendages  usually  show  traces  of  three  much 
reduced  digits. 

431.  Supplementary  Studies. — Allow  students  to  make  a  series  of  studies  of 
the  angle  made  by  the  axis  of  the  body  with  a  vertical  line  in  various  birds.  Com- 
pare this  angle  in  the  robin  when  at  rest  and  when  running.  Make  outline  drawings 
of  the  shank  and  toes  of  all  the  types  of  birds  which  can  be  found,  and  discuss  the 
differences  in  the  light  of  the  habits  of  the  birds.  Compare  these  with  figures  in 
texts.  Make  figures  of  the  varieties  of  webs  found  in  the  aquatic  birds. 

432.  Covering. — The  form  of  birds   as  outlined   above  is 
much  modified  by  the  presence  of  feathers.     They  increase  the 
26 


402  ZOOLOGY 

stretch  of  the  wings  and  the  surface  exposed  to  the  air,  and  thus 
are  important  as  aids  to  flight.  In  addition  they  are  protective 
in  several  respects.  The  pigment  possessed  by  the  feathers 
serves  to  enhance  greatly  the  beauty  and  variety  of  the  members 
of  the  group.  That  the  color  patterns  are  of  distinct  value  in 

FIG.  204. 


FlG.  204.    Diagram  showing  the  tracts  where  the  principal  growth  of  feathers  occurs  (Upupo  epops) 
From  Bronn.     The  dotted  areas  are  the  pterylce. 

Questions  on  the  figure. — Is  this  a  dorsal  or  a  ventral  view?  Find  a  figure 
giving  the  opposite  view  of  some  bird  and  compare  with  this.  Is  there  variety  in 
the  different  species  of  birds  as  to  the  distribution  of  the  growth  of  feathers? 

sexual  attraction  has  been  believed  by  many  naturalists.  The 
feathers,  together  with  the  scales  of  the  shank,  the  claws,  and 
the  beak,  are  epidermal  growths  and  represent  the  remnants 
of  the  exoskeleton  so  well  developed  in  some  of  the  lower  forms. 
Feathers  are  not  usually  produced  uniformly  over  the  body, 
but  are  grouped  in  regions  which  differ  in  different  species. 
They  also  vary  a  great  deal  in  form,  from  the  down  feathers  of 


AVES  403 

the  young  to  the  stiff  quill-feathers  of  the  wings  and  tail.  Most 
birds  shed  their  feathers  either  a  few  at  a  time  the  year  round  or 
within  a  short  period.  In  the  former  case  the  change  may  be 
scarcely  noticeable.  When  the  moulting  takes  place  rapidly,  it 
may  be  accompanied  by  profound  disturbance  in  the  health 
and  habits  of  the  animal.  The  new  feathers  may  differ  in  color 
from  the  old,  and  thus  a  periodic  change  is  apparent  in  the  dress 
of  some  of  our  birds.  This  is  not  infrequently  of  such  character 
as  to  accord  in  color  with  the  changes  in  nature  outside,  giving  a 
real  protective  value. 

433.  Supplementary  Topics. — In  what  various  ways  are  the  feathers  of  birds 
protective  ?  Explain  how  the  protection  is  realized  in  each  case.  What  varieties 
of  feathers  may  be  found  in  birds,  and  what  are  the  chief  differences  in  structure? 
How  are  the  color  patterns  obtained?  Are  they  made  up  of  feathers  of  one  color 
so  put  together  as  to  form  the  pattern,  or  is  a  single  feather  of  more  than  one  color? 
Does  a  single  feather  ever  show  an  independent  complex  color  pattern?  Where  is 
the  boundary  between  feathers  and  scales  on  the  legs  of  various  breeds  of  chickens? 
Do  you  find  any  evidences  that  feathers  are  highly  modified  scales?  Are  any  of 
the  feathers  like  the  hair  of  mammals? 

Secure  further  data  from  nature  and  from  reference  books  concerning  the 
moulting  habits  of  birds. 

434.  Endoskeleton. — The  chief  points  of  importance  to  the 
elementary  student  are  as  follows: 

1.  There  is  a  fusion  of  several  vertebrae  in  the  sacral  region 
(including  some  of  the  thoracic,  all  of  the  lumbar,  the  sacrals 
and  the  caudals)  with  the  dorsal  bones  of  the  pelvic  girdle,  to 
form  a  strong  dome-shaped  structure  above  the  viscera.     The 
cervical  vertebrae  vary  much  in  number  (eight  to  twenty-four) 
with  the  length  of  the  neck. 

2.  The  cranial  bones  fuse  closely,  and  the  bones  of  the  face 
are  prolonged  into  the  core  for  the  beak  (Fig.  219). 

3.  The  sternum  is  normally  well  developed  and  provided 
with  a  keel  to  which  the  muscles  of  flight  are  attached.     Finger- 
like  processes  also  increase  its  surface  for  the  attachment  of 
muscles  and  the  support  of  the  viscera. 

4.  The  ribs  are  double-headed,  and  each  has  a  process  on 
the  posterior  margin,  joining  it  to  the  rib  behind. 

5.  The  pectoral  girdle  has  its  clavicles  fused  ventrally  in  the 
flying  birds,  forming  the  "wish  bone." 


404  ZOOLOGY 

6.  In  the  pelvic  girdle  the  ventral  bones  (ischium  and  pubis) 
both  pass  backward  from  the  hip  joint  and  support  the  viscera. 

7.  The  ankle  region  of  the  birds  is  very  characteristic.     The 
proximal  tarsals  unite  with  the  tibia,  and  the  distal  tarsals  unite 
with  the  fused  metatarsals  to  form  the  tarso-metatarsus  or  shank. 
The   joint   is   between   the   proximal    and   distal   tarsals    (see 
Fig.   161). 

435.  Digestive  Organs. — The  horny  beak  entirely  replaces 
the  teeth  in  the  modern  birds.     In  the  early  members  of  the 
group  teeth  are  known  to  have  been  present.     The  esophagus, 
often  of  great  length,  is  usually  expanded  into  a  non-glandular 
crop,  where  the  food  is  stored  and  softened.     The  stomach  often 
consists  of  two  portions,  the  anterior  glandular  proventriculus 
and  the  posterior  muscular  gizzard.     In  birds  which  habitually 
feed  on  grains  or  other  hard  objects  the  inner  wall  of  the  gizzard 
is  lined  with  a  hard  and  thickened  cuticle  which  assists  in  grind- 
ing the  food.     Fragments  of  rock,  sand,  etc.,  are  nearly  always 
swallowed  by  grain-eating  forms  to  assist  in  the  process.     These 
are  manifestly  devices  to  do  work  usually  done  by  teeth.     The 
usual  glands  are  found  associated  with  the  digestive  tract,  ex- 
cepting the  salivary.     The  tract  ends  in  a  cloaca. 

436.  Supplementary  Studies. — What  is  the  exact  position 
of  the  crop  ?     Advantage  of  this  position  ?     Make  a  comparative 
study  of  the  beak  in  various  birds ;  how  adapted  to  the  habits  ? 
Is  there  any  recorded  evidence  that  the  character  of  the  gizzard 
in  a  given  individual  may  vary  somewhat  in  accordance  with 
the  food  used  ? 

437.  Respiration. — The  trachea  corresponds  in  length  to  the 
length  of  the  neck.     Its  rings  are  rigid  (ossified).     It  divides 
into  a  right  and  left  bronchus  which  pass  to  the  respective 
lungs.     The  lungs  are  closely  applied,  and  even  attached,  to  the 
dorsal  wall  of  the  thorax  and  are  small  in  proportion   to  the 
size  of  the  animal.     Some  of  the  bronchial  tubes  connect  with 
air  spaces  (nine  in  the  pigeon)  among  the  viscera  and  extending 
even  into  the  hollow  bones.     They  are  probably  chiefly  respira- 
tory in  function. 


AVES  405 

Bird  notes  are  produced  not  at  the  upper  end  of  the  trachea 
as  in  other  vertebrates  but  near  its  lower  end,  where  it  joins 
the  bronchi.  The  organ  is  called  the  syrinx.  Its  mode  of  action 
is  somewhat  similar  to  that  of  the  vocal  cords  in  the  larynx  of 
mammals. 

438.  The  Nervous  System  and  Organs  of  Special  Sense. — 

The  cerebral .  hemispheres  are  relatively  larger  than  in  any  of 
the  groups  yet  studied.  Their  surface  is  smooth.  The  cere- 
bellum is  also  large  and  concentrated  chiefly  in  a  central  or 
median  lobe.  By  the  growth  of  these  two  portions  the  well- 
developed  optic  lobes  are  crowded  into  a  lateral  position.  The 
olfactory  lobes  are  small  and  the  sense  of  smell  is  not  so  acute  as 
in  many  other  vertebrates.  The  optic  lobes  and  the  eyes  are 
well  developed  and  the  sense  of  sight  is  correspondingly  acute. 
The  eye  protrudes  as  a  somewhat  rounded  cone  in  front.  This. 
is  supported  by  a  ring  of  sclerotic  (bony)  plates.  The  power  of 
accommodation,  that  is,  of  focusing  the  eye  upon  objects  at 
different  distances,  is  very  great  in  birds.  In  addition  to  the 
upper  and  lower  lids  a  transparent  fold  of  the  conjunctiva 
(nictitating  membrane)  may  be  drawn  over  the  eye  from  the  inner 
corner.  Hearing  is  acute,  and  the  condition  of  the  ear  is  inter- 
esting chiefly  in  the  facts  of  the  absence  of  the  concha  of  the  ex- 
ternal ear,  and  in  the  presence  of  a  well-developed  but  uncoiled 
cochlea  in  the  internal  ear. 

The  birds  are  ''nervous"  animals.  That  is  to  say,  they  are 
sensitive,  active,  easily  aroused.  This  nervous  activity  is  cor- 
related with  the  high  temperature,  rapid  respiration,  and  large 
consumption  of  food. 

439.  Habits. — None  of  the  animal  groups  present  habits 
more  interesting,  more  readily  studied  or  more  suggestive  of 
the  adaptation  of  structure  to  the  demands  of  the  environment 
than  do  the  birds.     The  student  will  find  by  observation  and 
by  reference  to  current  works  on  natural  history  many  inter- 
esting facts  in  connection  with  bird-life.     Under  the  suggest- 
ive studies  a  partial  list  of  such  topics  will  be  found.     In  the 
chapter  on  Adaptations  and  in  the  section  on  the  classification 
of  birds  (Ch.  VIII,  and  §441)  additional  facts  have  been  pre~ 


406 


ZOOLOGY 


sented.     Much  of  the  time  given  to  the  practical  studies  of  the 
group  of  birds  should  be  directed  to  their  life  and  adaptations. 


FIG.  206. 


FIG.  205.  Diagram  of  the  female  genital  organs  of  a  Bird,  c,  cloaca;  *,  intestine;  k,  kidney; 
o,  ovary  with  ova  of  different  size;  od,,  oviduct;  o.f.,  funnel  of  the  oviduct;  o.o.,  opening  of  the  ovi- 
duct into  the  cloaca;  u,  ureter;  U.G.,  opening  of  ureter  into  the  cloaca.  Only  one  ovary  and  oviduct 
are  fully  developed  in  the  Birds. 

Questions  on  the  figure. — What  openings  has  the  oviduct?  Why  must  the 
union  of  sperm  and  ovum  take  place  before  the  egg  gets  well  down  the  oviduct? 
Define  the  cloaca.  On  which  side  are  the  sexual  organs  rudimentary  in  the  female 
bird? 

FIG.  206.  Diagram  of  the  urino-genital  organs  of  a  male  Bird,  ad.,  adrenal  body;  c,  cloaca; 
c.  intestine;  k,  kidney;  /,  testis;  u,  ureter;  M.O.,  opening  of  ureter  into  the  cloaca;  v.d.,  vas  deferens; 
v.d.o.,  opening  of  the  vas  deferens;  v.s.,  vesicula  seminalis. 

Questions  on  the  figure. — What  is  the  function  of  the  vas  deferens?  Of  the 
vesicula  seminalis?  What  differentiates  the  cloaca  from  the  intestine?  What 
are  the  chief  differences  in  the  excretory  organs  of  birds  and  mammals  ? 

These  various  habits  and  modes  of  life  have  frequently  been 
made  the  basis  of  classification:  for  example,  some  fly  and 
some  do  not;  some  wade,  having  long  legs;  others  swim  and 


AVES 


407 


have  webbed  feet;  some  capture  living  prey  with  talons  and 
curved  beak;  some  scratch  and  have  blunted  claws;  some  climb 
and  have  two  digits  directed  forward  and  two  backward; 
others  perch  and  have  only  one  toe  pointed  backward.  The 
resort  to  such  superficial  features  in  classifying  birds  suggests 
that  the  members  of  the  class  are  more  nearly  related  and  more 
similar  among  themselves  in  the  fundamental  features  of  struc- 
ture than  is  the  case  with  the  subdivisions  of  the  other  classes 
of  vertebrates. 

FIG.  207. 


g.w. 


FIG.  207.  Diagram  of  a  longitudinal  section  of  the  embryo  of  a  fowl,  without  the  amnion  and 
allantois.  Ectodermal  boundaries  are  in  continuous  lines,  the  entodermal  and  mesodermal  are  in 
broken  lines:  the  entodermal  of  short  dashes,  the  mesodermal  of  long,  b,  brain;  b.iv.,  body  wall; 
c.C.,  central  canal  of  spinal  cord;  co.,  coelom;  g,  gut;  g.w.,  wall  of  gut;  s.c.t  spinal  cord;  y.s.,  yolk  sac. 

Questions  on  the  figure. — What  is  the  relation  of  the  yolk  sac  to  the  digestive 
cavity  ?  Which  of  the  embryonic  layers  surrounds  it  ?  In  what  way  is  the  abun- 
dant yolk  in  the  yolk  sac  brought  into  the  circulation  of  the  embryo  (see  reference 
texts)  ? 

440.  Reproduction  and  Development. — Reference  has  al- 
ready been  made  to  the  fact  that  the  right  reproductive  organs 
of  the  female  birds  are  much  reduced  or  wanting.  The  ovum 
is  always  large,  containing  abundant  yolk.  When  mature  it 
breaks  from  the  ovary,  enters  the  funnel-shaped  end  of  the 
oviduct  and  as  it  passes  outward  is  fertilized.  It  then  receives 


4o8 


ZOOLOGY 


a  layer  of  albumen,  and  later  is  surrounded  by  a  membranous 
covering  and  by  a  porous,  limy  shell,  all  of  which  are  secreted 
by  the  walls  of  the  oviduct.  The  protoplasm  is  confined  to  a 
small  germinal  disc  and  segmentation  is  discoidal,  resulting  in 
a  blastoderm  like  that  of  reptiles.  In  the  newly  laid  egg  cleavage 

FIG.  208. 
aw.c  anf 


CO. 


al 


am* 


--co. 


FIG.  208.  Diagram  of  a  longitudinal  section  through  the  embryo  of  a  fowl,  showing  formation 
of  amnion  and  allantois  and  the  relation  of  these  membranes  to  the  embryo.  The  boundaries  are 
as  in  the  preceding  figure,  am1,  inner  or  true  amnion;  am2,  outer  or  false  amnion;  am.c.,  amniotic 
cavity;  al.,  allantois;  c.c.,  central  canal  of  the  spinal  cord;  co.,  ccelom;  g,  gut;  ys.,  yolk  sac. 

Questions  on  the  figure. — Which  of  the  three  embryonic  layers  enter  into  the 
amniotic  folds?  Which  go  to  form  the  allantois?  Show  that  the  cavity  between 
the  true  and  false  amnion  is  "extra-embryonic"  ccelom.  How  is  the  amniotic 
cavity  lined  ?  With  what  is  the  yolk  sac  lined  ?  The  cavity  of  the  allantois  is  in 
reality  a  portion  of  what  cavity?  Which  of  these  membranes  unite  in  mammals  to 
form  the  chorion  (see  §462). 

is  well  advanced.  After  the  egg  is  laid,'  cleavage  is  checked 
until  the  necessary  temperature  for  further  development  is 
supplied  either  by  the  brooding  of  the  parent  or  by  some  special 
device.  Owing  to  the  action  of  gravity  on  the  heavier  yolk 
the  living  disc  is  always  directed  upward, — the  position  most 
favorable  for  getting  the  warmth  of  the  parent's  body  in  incuba- 


AVES 


409 


tion.  For  the  details  of  further  development  the  student  must 
be  referred  to  more  extensive  texts,  but  it  may  be  stated  that 
the  blastoderm  comes  to  consist  of  two  layers  of  cells  which 
have  been  likened  to  two  watch  glasses  so  placed  as  to  enclose 
a  shallow  cavity.  The  outer  layer  is  ectodermal  and  is  continu- 
ous at  the  edge  with  the  inner,  which  is  composed  of  larger  cells 
incompletely  separated  from  the  yolk  beneath  (Fig.  13,  C,  4). 
This  inner  layer  gives  rise  to  both  entoderm  and  mesoderm.  The 
blastoderm  continues  to  grow  at  the  margins  until  the  yolk  is 
entirely  enveloped  by  a  living  membrane  which  is  well  supplied 
with  blood  vessels  and  serves  to  extract  the  food  for  the  use  of  the 
embryo  and  to  aerate  the  blood  before  the  lungs  become  of  use. 

FIG.  209. 


FIG.  209.     ArchcEopteryx  lithographica,  an  early  reptilian  Bird.     From  Claus. 

Questions  on  the  figure. — What  in  the  figure  shows  this  to  be  a  bird?  Wha* 
shows  it  to  be  different  from  typical  birds?  What  is  signified  by  each  of  the  term- 
in  its  scientific  name? 

The  amnion  and  allantois  (see  §423;  Fig.  208)  are  both  de- 
veloped as  in  reptiles.  Of  these  the  amnion  appears  first.  By 
a  study  of  Figs.  207  and  208,  together  with  others  in  the  reference 
texts,  it  will  be  seen  that  the  amnion  is  an  outgrowth  of  the  body 
wall  of  the  embryo  and  has  a  cavity  continuous  with  the  ccelom. 
The  outer  layer  is  known  as  the  false  amnion;  the  inner  is  the 
true  amnion  (Fig.  207,  am2,  am1).  Into  the  space  between  the 
amnion-layers  the  wall  of  the  gut  outpockets,  forming  the  al- 
lantois (Fig.  208,  a/.).  The  cavity  of  this  sac  is  continuous 
with  the  lumen  of  the  gut.  The  embryo  thus  becomes  com- 


4io 


ZOOLOGY 


pletely  surrounded  by  protective  membranes.  The  cavity 
between  the  true  amnion  and  the  body  wall  (Fig.  208,  am.  c.) 
is  the  amniotic  cavity  and  may  be  filled  with  a  fluid. 

441.  Classification  of  Aves. 

Subclass  I.  SaururcB  (reptile-tailed). — These  are  extinct  birds 
related  to  the  extinct  reptiles — the  dinosaurs — in  having  a 
vertebrated  tail,  and  jaws  bearing  teeth.  Each  vertebra  of 

FIG.  210. 


FIG.  210.     Apteryx  australis.     From  Romanes. 

Questions  on  the  figure. — What  peculiarities  does  this  bird  present?  What 
does  Apteryx  mean?  What  is  the  distribution  of  this  species?  What  are  its 
nearest  relatives  among  the  birds? 

the  tail  possessed  a  pair  of  feathers,  the  tail  thus  having  a  row 
of  rectrices  on  either  side. 

Archaeopteryx,  of  which  two  specimens  have  been  found  in 
the  lithographic  quarries  of  Bavaria,  represents  the  group  and 
was  about  the  size  of  a  crow  (Pig.  209). 

Subclass  II.  Neornitkes  (modern  birds). — This  group  is 
characterized  by  the  reduction  and  fusion  of  the  tail  vertebrae 
in  such  a  way  that  the  tail  feathers  (rectrices)  are  arranged 
in  a  semicircle  (or  sometimes  wanting).  Teeth  are  wanting 
except  in  some  extinct  forms,  which  stand  intermediate  between 
the  Saururae  and  the  recent  birds. 

Division  I.  Ratitcs  (flat). — These  are  running  birds  with  a 


AVES 


411 


flat  breast  bone  (i.e.,  no  keel)  and  with  all  the  organs  of  flight 
much  reduced.  The  barbs  of  the  feathers  are  not  held  together 
by  barbules,  thus  producing  plumes. 

The  Ratitas  (order  Cur  sores  or  runners)  are  the  lowest  forms 
of  living  birds  and  include  the  ostriches,  emus,  cassowaries,  in 
all  of  which  the  wings  are  reduced,  and  the  Apteryx  or  wingless 
bird  of  New  Zealand  (Fig.  210)  in  which  they  are  very  rudi- 

FlG.    211. 


FIG.  211.     Ostrich  (Struthio).     From  Wood's  Natural  History. 

Questions  on  the  figure. — Which  of  the  types  of  feathers  of  ordinary  birds 
become  the  plumes  in  the  ostrich  ?     What  is  the  real  size  of  the  ostrich  ? 


mentary.  The  ostrich  (Fig.  211)  is  the  largest  and  most  power- 
ful of  living  birds.  Ostriches  are  somewhat  gregarious,  and 
frequent  regions  more  or  less  desert.  At  mating  time  they 
unite  in  pairs,  the  male  assisting  in  incubating  the  eggs,  which 
are  laid  in  holes  in  the  sand.  Ostrich  culture  is  an  important 


412  ZOOLOGY 

industry  in  South  Africa  and  to  a  certain  extent  in  America, 
on  account  of  the  plumes  which  are  extensively  used  as  orna- 
ments. Besides  the  types  mentioned  there  are  a  number  of 
extinct  forms  belonging  to  this  division,  some  of  which  have 
become  extinct  in  recent  time.  Mpyornis  is  one  of  these, 
formerly  a  native  of  Madagascar,  where  remnants  of  its  eggs 
have  been  discovered  showing  that  its  volume  was  about  six 

FIG.  212. 


FIG.  212.     Wood  Duck  (Aix  sponsa).     Photographed  by  Dr.  J.  W.  Folsom. 

times  that  of  the  ostrich  egg,  i.e.,  having  a  capacity  of  about 
two  gallons. 

Division  II.  Carinatce  (with  a  keel). — Birds  with  the  keeled 
breast  bone,  the  wings,  and  the  other  organs  of  flight  usually 
well  developed.  Barbs  of  the  feathers  have  barbules.  All  the 
modern  flying  birds  are  embraced  in  this  subclass. 

The  further  subdivisions  of  the  Carinatse,  as  given  in  the 
recent  classifications,  based  upon  internal  structure,  are  entirely 
unsuited  for  beginners.  An  older  arrangement  of  the  prin- 
cipal orders,  based  upon  habits  and  certain  superficial  features, 
is  presented  below  for  the  convenience  of  the  student.  It 


AVES 


413 


should  be  remembered,  however,  that  the  classification  is  not 
the  best  possible,  inasmuch  as  forms  in  reality  not  very  closely 
related  in  structure  are,  according  to  it,  placed  together  because 
of  similar  habits.  The  student  is  asked  to  refer  to  other  texts 
for  different  arrangements. 

FIG.  213. 


PIG.  213. — Ross1  Gull  (Rhodostethia  rosea).     Upper  figure  adult  male;  lower,  young  female.     From 
"Chapters  on  Natural  History";  drawn  by  Dr.  R.  W.  Shufeldt  after  Ridgway. 

Questions  on  the  figure. — What  indications  of  structural  adaptation  to  habits 
do  you  find  in  the  figure?     What  sexual  dimorphism  is  perceptible? 

The  Pygopodes  (feet  on  the  rump)  include  the  auks,  penguins, 
grebes,  and  loons.  These  are  all  aquatic  birds  and  are  expert 
divers  and  swimmers.  Their  feet  are  poorly  adapted  for  the 
land,  and  in  consequence  the  birds  are  awkward.  The  auks 
and  penguins  have  poorly  developed  wings.  The  loons  however 
are  good  fliers.  The  penguins,  auks,  and  puffins  are  marine 


414 


ZOOLOGY 


forms.  The  loons  and  grebes  are  found  in  fresh  waters.  The 
penguins  at  the  breeding  season  are  gregarious  and  are  often  so 
thick  as  literally  to  cover  the  surface  that  is  level  enough  to 
be  occupied.  They  lay  one  or  two  eggs,  with  no  pretense  of 
making  a  nest.  The  loons  and  grebes  may  use  flags  and  rushes 
for  nests. 

FIG.  214. 


FIG.  214.     Green  Heron  (Ardea  virescens).     Photographed  by  Dr.  R.  W.  Shufeldt. 

Questions  on  the  figure. — To  what  order  of  birds  does  the  heron  belong? 
What  are  its  nearest  relatives?     What  can  you  say  of  the  habits  of  the  order? 

The  Longipennes  (long-winged)  are  the  albatross,  the  tern, 
gulls,  petrels,  shearwaters,  etc.  These  are  aquatic  birds  with 
webbed  feet  and  long,  pointed  wings.  They  are  splendid  fliers 
and  mostly  good  swimmers.  There  are  both  marine  and  fresh- 
water types.  They  are  gregarious,  especially  at  the  breeding 
season,  when  they  swarm  on  sandy  shores,  in  the  marshes,  or 
on  the  rocky  coasts,  where  they  lay  their  eggs  in  crude  nests  or 
on  the  bare  rocks.  They  are  the  most  common  birds  of  the 
seashore  and  the  high  seas. 


AVES 


415 


In  the  Steganopodes  (web-footed)  are  included  cormorants, 
pelicans,  flamingos,  storks,  the  ibis,  etc.  These  are  large  water 
birds  in  which  the  legs  are  usually  long  and  the  feet  suited  to 
wading  and  swimming.  They  are  heavy  eaters,  living  largely 
on  fish.  They  are  often  expert  fishermen, — the  white  pelicans 
acting  in  concert  in  driving  the  fish  before  them  into  shallow 
water,  where  they  capture  numbers  of  them  in  a  capacious  pouch 

FIG.  21=;. 


FIG.  215.     A  right  lateral  view  of  the  skull  of  the  American  Flamingo  (Phcenicopterus  ruber). 
Photographed  from  specimen  by  Dr.  R.  W.  Shufeldt. 

Questions  on  the  figure. — Distinguish  upper  and  lower  jaws,  comparing 
them  as  to  massiveness.  Is  this  the  usual  condition  in  birds?  How  much  of  the 
skull  is  occupied  by  the  brain  ?  To  what  habits  of  the  flamingo  is  the  form  of  its 
beak  an  adaptation?  Compare  with  Fig.  219. 

of  the  lower  jaw.     The  flamingos  are  long-legged,  wonderfully 
colored  birds  with  most  interesting  gregarious  nesting  habits. 

The  Anseres  (geese).  The  geese,  ducks,  and  swans  are 
familiar  because  of  their  widespread  migrations  and  because  of 
numerous  domestic  varieties.  They  have  heavy  bodies  and: 
characteristic  form.  Three  toes  are  united  by  the  web.  Their- 


416  ZOOLOGY 

bills  are  broad  and  serrate.  They  feed  upon  vegetation  and  the 
smaller  water  animals.  Their  wings  are  long  and  broad,  and 
they  are  good  fliers.  The  ducks  and  geese  spend  their  winters 


FIG.  216. 


X  ^.-\v 

_V|P        > 

I 


FIG.  216.     Pelican  (Pelecanus  erythrarhynchus).     By  Folsom. 

Questions  on  the  figure. — What  is  the  nature  and  purpose  of  the  fold  beneath 
the  jaw?     To  what  division  of  the  birds  does  the  pelican  belong? 

in  the  tropics  and  nest  in  the  colder  regions.  They  are  much 
hunted  by  man  during  these  migrations,  when  they  frequent  the 
streams  and  lakes  of  the  temperate  regions.  They  are  found 
the  world  over.  There  are  said  to  be  forty  species  of  ducks, 


AVES  417 

sixteen  species  of  geese,  and  three  species  of  swans  native  to 
North  America. 

PaludicolcB  (living  in  swamps).  This  group  includes  marsh 
birds  which  stalk  about  the  shallow  waters  picking  up  the  small 
animals  to  be  found  there.  They  have  long  legs  and  corre- 
spondingly long  necks.  The  cranes,  trumpeters,  rails,  coot, 
tern,  and  gallinules  are  classed  here.  Their  toes  are  not  com- 

FIG.  217. 


FIG.  217.     Ruffed  Grouse  (Bonasa  umbellus).     Photographed  by  J.  W.  Folsom. 

pletely  webbed.     They  are  light  birds  in  proportion  to  the  reach 
of  their  neck,  legs,  and  wings. 

LimicolcB  (shore-residents)  inhabit  the  margins  of  streams 
and  lakes  and  may  wade  in  the  shallows.  These  are  similar 
in  many  ways  to  the  preceding  group,  but  are  smaller  birds. 
The  group  embraces  the  snipes,  woodcock,  sand-pipers,  plover, 
killdeer,  avocets,  and  many  others.  They  have  slender  legs 
and  long  bills.  They  are  alert  birds  with  quick,  active  habits, 
27 


4i8 


ZOOLOGY 


AVES 


419 


protectively  colored,  and  take  promptly  to  cover.     For  these 
reasons  they  are  "game"  birds,  sought  by  hunters. 

Raptores  (robbers)  are  preying  birds  with  hooked  beak  and 
claws,  sharp  vision,  powerful  wings,  and  instincts  to  match. 

FIG.  219. 


PIG.  219.     Skulls  of  gallinaceous  Birds,  as  Partridge,  Grouse,  etc.      Photographed  from  the  speci- 
mens by  Dr.  R.  W.  Shufeldt.     Adult.     J  natural  size. 

Questions  on  the  figures. — Compare  these  skulls  and  note  the  points  of  simil- 
arity and  dissimilarity.  Find  the  position  of  eye,  ear,  and  nares.  What  are  the 
chief  points  of  contrast  between  these  skulls  and  that  of  the  owl  (Fig.  222),  and  of 
the  flamingo  (Fig.  215)? 

Eagles,  hawks,  kites,  falcons,  owls,  and  vultures  may  be  classed 
here.  They  are  widely  distributed,  and  have  a  great  range  of 
size  and  adaptations.  The  vultures,  including  the  buzzards 


420 


ZOOLOGY 


and  the  great  condor  of  South  America,  are  scavengers.  The 
owls  are  noted  for  their  striking  appearance  and  for  their 
nocturnal  habits  and  the  adaptations  associated  therewith. 


FIG.  220. 


PIG.  220.     Great  horned-owl  (Bubo  virginianus).     Adult  female.     Photographed  from  life  by  Dr. 

R.  W.  Shufeldt. 

Questions  on  the  figure. — What  are  the  habits  of  owls?  Does  the  figure  show 
any  structural  adaptations  to  known  habits?  How  does  the  standing  position  of 
the  owl  differ  from  that  of  other  birds  of  your  acquaintance?  See  also  Figs.  221 
and  222. 

There  is  a  strong  human  prejudice  against  the  group,  partly 
because  some  of  the  members  attack  our  poultry  and  other 
domestic  animals.  Close  observation  shows,  however,  that  most 


AVES  421 

of  the  species  are  either  harmless  or  actually  helpful  because 
they  destroy  noxious  rodents.  Cooper's  hawk,  the  sharp- 
shinned  hawk,  and  the  great  horned  owl  are  real  pests. 

The  GallincB  (fowls)  include  quail,  partridges,  grouse,  pheas- 

FlG.    221. 


PIG.  221.     Great  horned-owl  (Bubo  virginianus).  Young.     Photographed  from  life  by  Dr.  R.  W. 

Shufeldt. 

Questions  on  the  figure. — Compare  the  young  at  all  points  with  the  adult. 
What  are  the  points  of  difference?     Of  manifest  likeness? 

ants,  turkeys,  peafowls,  and  the  various  kinds  of  chickens. 
These  birds  have  a  plump,  characteristic  form.  The  legs  are 
mostly  short  or  of  medium  length  and  armed  with  short,  blunt 
claws.  The  beak  is  stout  and  bent  slightly  downward  at  the 


422 


ZOOLOGY 


point.  As  a  rule  they  do  not  take  long  flights.  They  nest  on 
or  close  to  the  ground  and  they  get  their  food  there.  They  are 
less  highly  organized  nervously  than  many  birds,  and  more 
readily  tamed.  Their  habits  of  life  furthermore  fit  them  for 
domestication.  They  have  large  broods,  and  in  early  life  these 

FIG.  222. 


PlG.   222.     Skull  of  Owl   (Syrnium   iifl>nh>snni).     After  ShufoKlt.  photographed  from  specimens. 
Upper  figure  bisected,  showing  brain-case;  the  lower  from  a  dorsal  aspect. 

Questions  on  the  figure.— Is  the  owl  a  bird  of  prey?  What  is  the  position  of 
the  eyes  in  relation  to  the  skull?  Of  the  nares?  Compare  these  figures  with  the 
head  of  the  owl  (Fig.  220),  and  with  the  skulls  in  Pig.  219. 

follow  the  mother  bird  closely.  This  instinct  may  linger  into 
adult  life  and  families  often  remain  together  in  ''coveys" 
throughout  the  first  year,  or  even  longer.  Thus  large  flocks  may 
be  formed.  Their  secretive  instincts,  their  protective  markings, 
and  their  edible  flesh  make  them  "game"  birds  of  first  quality. 


AVKS 


423 


Before  civilized  man  destroyed  their  haunts  and  persistently 
hunted  them  with  his  improved  weapons,  these  were  among  the 
very  successful  wild  birds.  Most  species  are  now  really  quite 
scarce.  The  only  hope  of  preserving  them  is  by  legal  protection 
at  critical  times  or  by  domestication. 

Chief  among  the  domesticated  birds  is  the  common  fowl 
which  is  descended  from  a   species   native   to    southern  Asia 

Fi*.   223. 


FIG.  223.     Belted   Kingfisher   (Ceryle  alcyon,   L)..     About   one-fourth   natural  size.     By  J.   W. 

Folsom. 

(Gallus  bankiva).  By  breeding  and  selection  almost  innumer- 
able varieties  have  been  produced.  The  poultry  and  egg  in- 
dustry is  one  of  the  most  important  in  the  country,  amounting 
to  more  than  700,000,000  dollars  annually.  In  1901,  one  and 
one-half  billion  dozens  of  eggs  (estimated)  were  produced  on 
the  farms  of  this  country.  This  in  one  of  the  great  sources  of 
human  food.  Turkeys,  guinea-fowl,  and  pea-fowl  contribute 
in  less  degree. 

The  Columbce  (doves)  are  in  many  respects  similar   to  the 


424 


ZOOLOGY 
FIG.  224. 


FlG.  224.     Arctic  three-toed   Woodpecker.     From  U.   S.   Dept.   Agriculture,   "North  American 

Fauna." 

FIG.  225. 


FIG.  225.     Yellow-billed  Cuckoo  (Coccyzus  americanus).     Adult  male.     Photographed  from  life  by 

Dr.  R.  W.  Shufeldt. 

Question  on  the  figure. — What  are  the  nearest  relatives  of  the  cuckoos  among 
the  birds? 


AVES 


425 


FIG.  226. 


FIG.  226.     Clark's  Crow.     U.  S.  Dept.  Agriculture:   "North  American  Fauna." 


FIG.  227. 


\ 


FIG.   227.      Nestling  Crows  (Corrus).     From   U.  S.   Dept.  Agriculture  Year-book,   1900. 


426 


ZOOLOGY 


Gallincs.  With  the  doves  are  included  the  pigeons.  About  a 
dozen  species  are  found  in  America.  None  of  these  species  is 
very  numerous.  A  century  ago  the  passenger  pigeon  occupied 
the  region  east  of  the  Mississippi  River  in  great  flocks  number- 
ing millions  of  individuals.  In  the  hope  of  saving  the  species 
there  has  been  for  some  years  a  standing  offer  of  consider- 
able sums  of  money  for  information  of  a  nesting  pair  of  this 
species.  The  domestic  pigeon,  in  its  numerous  varieties,  has 
arisen  by  breeding  and  selection  from  the  blue  rock  pigeon  of 


FIG.  228.     Gold-finch  (Spinus  tristis).     U.  S.  Dept.  Agriculture  Year-book,  1898. 

the  Old  World.  In  cross  breeding  we  frequently  get  "rever- 
sions ' '  to  this  type,  showing  that  the  characters  are  being  carried 
in  the  germ  plasm  even  when  the  combinations  were  not  such  as 
to  make  them  appear  in  the  body. 

In  this  group  the  young  are  fed  with  food  digested  by  the 
parent  and  regurgitated  into  the  mouth  of  the  helpless  young. 

The  Pici  (woodpeckers)  have  two  toes  directed  forward  and 
two  backward  in  adaptation  to  the  position  they  take  in  climb- 
ing. The  tail  may  have  stiff,  pointed  feathers  with  which  they 


AVES 


427 


FIG.  229. 


FIG.  229.     Mocking  Bird   (Mimus   polyglottos).     From   Dept.   Agriculture  Year-book,    1895. 


FIG.  230. 


FlG.  230.     Wood    Thrush    (Hylocichla    mustelina).     Female,    one-half   size.     By   J.    W.   Folsom. 


428 


ZOOLOGY 


anchor  when  pecking  or  climbing.  They  have  strong,  sharp, 
chisel-like  bills.  By  means  of  these  they  get  at  insects  under  the 
bark  of  trees,  dig  into  the  tender  cambium  of  plants,  and  even 
excavate  cavities  for  nests  in  the  timber.  With  their  long, 
protrusible  tongue  they  pick  up  insects  routed  out  by  the 
hammering  of  their  beaks.  The  tapping  sound  is  also  a  means  of 
attracting  mates.  Some  species  bury  acorns  in  holes  they  have 
made  in  the  trees,  returning  for  them  when  food  supplies  are 
low. 

FIG.  231. 


FIG.  231.     The   Meadow  Lark.     From   U.   S.   Dept.   Agriculture   Year-book,    1895. 


They  are  striking  birds,  often  most  interestingly  colored. 
Our  most  common  forms  are  the  red-headed  woodpecker,  the 
downy  woodpeckers,  the  flicker,  the  yellow-bellied  sapsucker. 
The  latter  is  an  injurious  species,  as  it  opens  trees  to  decay  and 
attacks  of  fungi,  and  probably  spreads  such  diseases.  The  rest 
help  keep  in  check  various  insect  pests. 

Near  the  woodpeckers  are  often  classed  the  cuckoos,  king- 
fishers, and  toucans  or  hornbills.  The  kingfisher  and  rain  crows 
or  American  cuckoos  are  likely  to  come  under  the  observation 


AVES 


429 


of  the  student.  Some  species  of  cuckoos  lay  their  eggs  in  the 
nests  of  other  birds,  where  they  are  hatched  and  fed  by  the  host. 
In  return  for  this  they  are  said  often  to  cast  the  young  of  the 
proper  owners  out  of  the  nest. 

The  chimney-swifts,  whippoorwill,  and  the  humming  birds 
are  somewhat  intermediate  between  the  forms  just  spoken  of 
and  the  next  order. 

The  Pas  seres  (sparrow-like)  is  the  greatest  order  of  birds. 
It  has  more  than  sixty  recognized  families  and  more  than  6,000 

FIG.  232. 


FIG.  232.     Loggerhead  Shrikes  (Lanius  ludovicianus).     By  J.  W.  Folsom. 

species.  About  twenty  to  twenty-five  of  these  families  are  found 
in  North  America.  These  birds  are  mostly  small,  with  three 
toes  in  front  and  one  behind,  and  adapted  to  perching.  The 
majority  are  gifted  with  some  powers  of  song  (oscines).  A 
smaller  division,  of  which  the  king  bird  and  phcebe  may  be  taken 
as  types,  are  known  as  chattering  or  crying  birds  (clamatores) . 
Among  the  families  of  oscines,  which  the  student  is  most 


430  ZOOLOGY 

likely  to  meet,  are:  the  crow  family  (Corvida;  Fig.  226)  including 
crows,  and  blue  jays;  the  sparrows  and  finches  (FringillidcB), 
including  also  snow  birds,  crossbills,  and  grosbeaks;  the  oriole 
family  (Icterida)  including  orioles,  bobolinks,  meadowlark,  and 
the  blackbirds;  the  warblers  (Mniotiltida) ,  small  migrating 
birds  with  bright  colors  and  most  interesting  and  attractive 
qualities;  the  wrens  (Troglodytida) ;  and,  finest  of  all,  the  thrush 
family  (Turdidoz),  including  the  various  thrushes,  the  robin, 
bluebirds,  and  many  others. 

For  further  description  of  the  numerous  interesting  families 
of  Passeres  the  student  must  refer  to  some  special  book  on 
birds.  The  study  of  their  habits  and  form  constitutes  one  of 
the  most  popular  and  entertaining  subjects  of  natural  history 
for  the  recreation  studies  of  busy  people.  Much  good,  and 
some  very  indifferent,  literature  intended  for  guidance  in  such 
studies  is  now  being  produced. 

442.  Relation  to  Nature. — It  is  easy  to  be  seen  from  the  fore- 
going discussion  that  birds,  while  very  much  alike  in  funda- 
mental structure,  have  succeeded  in  adjusting  themselves  to  a 
remarkably  varied  life.  In  structure  this  adaptation  is  shown 
in  the  size  and  shape  of  the  body,  in  the  modifications  of  such 
external  structures  as  neck,  beak,  legs,  toes,  claws,  wings,  and 
tail.  Among  the  instincts  and  habits  the  following  have  a  large 
place  in  the  adaptations  of  birds:  the  food  habits,  instincts  of 
song,  nesting  and  breeding  instincts,  care  of  young,  and  response 
to  climatic  changes  as  shown  in  the  migration  impulses. 

Wings  vary  from  the  most  rudimentary  remnants,  as  in  the 
ostrich  and  apteryx  (kiwi),  to  the  powerful  ones  of  ducks  which 
drive  the  birds  at  a  rate  of  ninety  miles  an  hour.  The  tail  is 
equally  variable.  There  may  be  no  special  tail  feathers,  as  in 
the  kiwi,  or  the  tail  may  be  of  the  most  striking  character,  as 
in  the  male  peafowl  or  bird  of  paradise.  In  these  extreme  styles, 
as  in  many  birds  of  smaller  size,  the  tail  is  a  sex  development, 
the  male  usually  having  greater  specialization.  Tails  of  more 
normal  size  serve  as  balancers  when  the  birds  are  perched  and 
as  rudders  when  they  fly. 

The  legs  and  neck  are  closely  related  in  length.     If  the  legs 


AVES  431 

are  long  the  neck  must  be  of  corresponding  length  to  allow  the 
head  to  reach  the  ground.  Long  legs  are  often  coupled  with  a 
wading  habit;  though  in  the  ostrich  and  some  of  the  land  birds 
length  is  associated  with  the  running  habit. 

The  feet, — including  the  number  and  position  of  toes,  the 
character  of  the  claws,  and  the  presence  of  the  web, — are  a  good 
index  of  the  manner  of  life.  The  amount  and  arrangement  of 
the  web  varies  greatly  in  the  different  water  birds.  The  beak 
and  the  feet,  though  separated  by  the  extreme  length  of  the 
animal  are  really  three  appendages  which  work  together.  Aside 
from  the  prime  functions  of  the  feet  in  walking  and  swimming, 
the  beak  and  claws  work  in  close  cooperation  in  getting  food,  in 
offense  and  defense,  and  in  the  nest-building  operations.  Both 
claws  and  beak  are  strikingly  adapted  to  the  food  habits. 

By  means  of  the  syrinx  (§437)  most  birds  produce  sounds. 
In  the  higher  birds  (oscines)  the  sounds  may  become  highly 
organized  and  melodious.  Such  songs  are  for  the  most  part 
related  to  mating  and  the  breeding  season.  The  call-notes  are 
more  permanent  though  not  so  full  of  melody.  By  these  the 
birds  express  such  simple  emotional  states  as  fear,  anger,  con- 
tentment, and  excitement.  They  are  a  simple  language  well 
understood  by  the  other  members  of  the  species.  It  is  probable 
that  the  mating  songs  are  similarly  expressions  of  the  exuberant 
emotions  of  the  mating  time,  which  become  sustained  and 
elaborated  from  the  pleasure  they  come  to  give  the  producer. 
In  other  words  they  represent  a  passage  from  the  useful  to  the 
esthetic  even  for  the  bird  himself. 

The  succession  of  instincts  which  in  the  spring  will  cause 
even  one-year-old  birds  to  go  with  remarkable  precision  through 
the  migration,  the  selection  of  a  suitable  place  and  materials 
for  nest  making,  the  mating,  egg-laying,  incubation,  feeding 
and  protecting  the  young,  and  finally  their  introduction  to  the 
world,  is  at  once  the  marvel  and  despair  of  the  naturalist.  In 
all  these  respects  there  is  the  greatest  variation.  Some  mate 
yearly,  others  mate  for  life.  Some  conceal  their  nests  with 
greatest  ingenuity  and  build  it  with  great  care,  while  others  lay 
their  eggs  in  the  barest  possible  pfaces.^  They  may  lay  one  egg 
or  a  score.  The  hen  may  incubate  the  eggs  or  the  male  may. 


432  ZOOLOGY 

Sometimes  both  take  part.  The  cock  may  supply  the  hen  with 
food  during  incubation  or  pay  no  attention  to  the  process.  The 
young  may  be  ready  to  leave  the  nest  and  run  about  as  soon  as 
they  are  hatched  and  dry  (precocious)  or  they  may  be  naked 
and  helpless  for  days  (altricial). 

The  coloration  of  birds  is  most  varied  and  interesting.  They 
are  to  the  vertebrates  what  the  insects  are  to  the  invertebrates. 
As  in  insects,  the  color  is  due  to  pigment,  to  physical  surface 
markings  which  produce  luster,  or  to  both.  Males  and  females 
are  often  differently  colored ;  if  so  the  males  are  the  more  striking. 
Young  birds,  before  the  sex  qualities  appear,  are  more  like  the 
females. 

Some  birds  are  differently  colored  at  different  seasons.  Where 
there  is  a  difference  the  more  highly  colored  feathers  are  pro- 
duced in  the  winter  and  spring  as  the  mating  period  approaches. 
Color  in  birds  is  believed  by  some  to  be  due  to  the  high  met- 
abolic activity  they  show.  This  activity  is  believed  to  pro- 
duce pigments  as  waste  products.  These  come  to  be  represented 
in  the  coloration  of  the  feathers.  The  metabolism  is  more  active 
at  the  mating  season.  Color  may  be  useful  to  birds  in  two  ways. 
It  may  serve  as  recognition  marks  by  which  members  of  the 
species  may  quickly  recognize  each  other,  and  possibly  prove 
attractive  to  mates;  and  in  some  instances  at  least  it  serves  for 
concealment  because  of  likeness  to  the  surroundings. 

The  eggs  of  birds  are  frequently  remarkably  colored.  This 
color  is  in  the  calcareous  secreted  shell.  It  is  due  to*  color  se- 
creted by  certain  cells  in  the  oviduct.  The  color  of  eggs  of  a 
species  is  often  as  distinctive  as  the  color  of  the  bird.  It  may 
be  uniform,  or  in  definite  spots,  or  in  irregular  splotches.  It  is 
probable  that  the  coloration  of  eggs  is  also  frequently  protective. 

None  of  the  bird's  responses  to  its  conditions  is  more  striking 
than  the  act  of  migrating.  Some  do  not  migrate,  but  most  do. 
The  value  of  the  instinct  is  clear  enough.  Each  species  has  its 
own  customs.  The  time  of  starting,  the  route,  the  rate  of  migra- 
tion, and  the  termini  vary  for  different  species.  Usually  the 
rate  is  not  rapid.  Some  fly  pretty  steadily  for  considerable 
distance.  The  ducks  work  their  way  along  the  streams  and 
lakes,  or  from  one  to  another.  The  warblers  flit  from  forest  to 


AVES  433 

forest,  from  tree  to  tree,  or  along  the  fence  rows,  living  on  the 
country.  Sometimes  it  is  just  a  slow  retreat  before  the  advanc- 
ing cold  or  progress  with  the  advancing  spring.  At  other  times 
it  may  involve  long  flights. 

The  golden  plover  illustrates  an  extreme  case.  They  breed 
in  the  arctic  circle  in  June  and  July.  They  "winter"  south  of 
the  equator  in  South  America  from  September  to  March;  that 
is  to  say  during  the  southern  summer.  Their  course  in  the  north- 
ward spring  migration  is  not  fully  known,  but  it  is  believed  that 
they  follow  an  overland  route  over  northern  South  America, 
Central  America,  Mexico,  Central  United  States  and  Canada 
to  their  Arctic  breeding  grounds.  The  return  journey  south- 
ward is  better  known.  From  their  breeding  grounds  they  go  to 
Labrador  in  August.  Here  they  feed  on  berries  and  grow  fat 
after  their  relative  fast.  They  work  their  way  down  to  the  coast 
of  Nova  Scotia.  Here  they  desert  the  land  and  fly  across  the 
ocean  to  the  north  shoulder  of  South  America.  They  may  stop 
temporarily  in  the  islands  of  the  West  Indies ;  or  they  may  make 
the  whole  distance  apparently  without  stop.  In  the  course  of  a 
few  weeks  they  cross  the  equator  and  reach  their  southern- 
most terminus  in  Argentine.  Some  other  species  have  been 
followed  with  equal  care  and  have  almost  equal  range. 

Birds  have  many  enemies  among  animals.  Some  of  the  birds 
are  prey  to  other  birds.  Snakes,  some  lizards,  and  many  of  the 
smaller  mammals  feed  upon  them  whenever  they  can.  Many 
eggs  are  eaten  by  these  and  other  animals.  During  the  nestling 
stage  birds  are  in  especial  danger.  The  parents  themselves  are 
often  taken  at  this  time.  Man  himself  has  probably  proved  the 
worst  enemy  of  the  birds,  which  is  scarcely  wise  or  fair,  since  the 
birds  are  among  his  best  friends. 

443 .  Relations  of  Birds  to  Man. — Man  has  directly  destroyed 
many  of  the  wild  birds  for  food,  for  their  plumage,  for  sport,  or 
because  he  deemed  them  injurious  to  his  interests.  In  an  in- 
direct way  he  has  destroyed  even  more.  Many  of  the  birds  do 
not  take  kindly  to  civilization,  but  prefer  the  quiet  of  unchanged 
nature  for  feeding  and  breeding.  Such  birds  have  been  gradu- 
ally pushed  into  more  limited  or  less  favorable  territories. 
28 


434  ZOOLOGY 

Others  have  adjusted  themselves  to  human  surroundings  and 
to  our  crops  of  grains  and  fruits.  These  have  suffered  less; 
indeed  some  of  them  have  larger  numbers  than  would  be  possible 
in  the  wild  state.  Still  others  have  been  domesticated,  and  by 
breeding,  selection,  and  support  have  been  greatly  changed  and 
improved  for  human  uses.  The  birds  that  have  been  domesti- 
cated are  chiefly  the  ostriches,  members  of  the  goose  family 
(ducks,  geese,  swans),  several  Gallinae  (chicken,  turkey,  guinea- 
fowls,  pheasants,  pea-fowls)  a  few  of  the  Rap  tores  (as  the  falcon), 
the  pigeons,  and  occasional  members  of  other  orders,  as  parrots, 
magpies,  and  occasional  song  birds. 

The  student  of  Zoology  should  not  fail  to  familiarize  him- 
self with  the  main  types  of  chickens  which  have  been  developed. 
A  few  of  the  main  classes  of  breeds  are  as  follows:  the  Asiatic 
breeds,  including  Brahmas,  Cochins,  Langshans;  the  Mediter- 
ranean breeds,  including  Leghorns,  Minorcas,  and  Black  Spanish; 
the  Dutch  breeds,  Hamburgs  and  Red-caps;  the  French  breeds, 
Houdans,  and  Crevecceurs;  the  English  breeds,  as  Orpingtons 
and  Dorkings;  the  American  breeds,  as  Plymouth  Rocks,  Wyan- 
dottes,  Dominiques,  and  Rhode  Island  Reds;  and  fancy  breeds 
as  the  Bantams,  game  birds,  etc. 

The  special  egg  breeds  are  the  varieties  of  Leghorns,  Minorcas, 
Spanish,  and  Red-caps.  The  meat  producing  breeds  are  Bramas, 
Cochins,  and  Langshans.  The  Plymouth  Rocks,  Wyandottes, 
Rhode  Island  Reds,  Orpingtons,  and  Dorkings  are  desirable 
breeds  for  general  service  in  both  respects. 

Aside  from  the  food  values  of  poultry,  the  group  of  birds 
renders  man  most  notable  service  in  two  ways, — by  destroying 
noxious  insects  and  by  eating  weed  seeds.  To  be  sure  they  do 
take  some  toll  in  cultivated  grains  and  in  fruits;  but  with  very 
few  exceptions  it  has  been  shown  by  examination  of  stomach 
contents  of  various  species  at  various  ages  that  the  great  bulk 
of  the  smaller  birds  are  helpful  to  human  interests,  and  often 
greatly  so.  An  observer  found  7,500  seeds  of  one  common  weed 
in  the  stomach  of  one  dove.  If  all  birds  were  suddenly  destroyed, 
unquestionably  many  species  of  insects  would  at  once  increase 
to  the  point  where  they  would  be  a  fearful  pest  to  mankind, 
before  a  new  balance  in  nature  would  be  struck.  Mice  and 


AVES  435 

other  hurtful  rodents  are  captured  by  owls,  hawks,  and  other 
preying  birds. 

These  facts  are  at  the  bottom  of  the  recent  agitation  that 
egg  collecting  and  the  slaughter  of  birds  for  sport  shall  stop.  A 
careful  investigation  after  seven  years  of  close  study  gives  the 
following  list  of  birds  which  do  more  injury  than  good;  the  Eng- 
lish sparrow,  Cooper's  hawk  and  the  sharp-shinned  hawk,  the 
sapsucker,  and  the  crow.  Recent  investigations  show  that  the 
robin  is  the  most  numerous  bird  in  the  United  States,  averaging 
six  pairs  to  every  farm  of  58  acres.  The  English  sparrow  comes 
second  with  five  pairs  to  that  area.  For  every  100  robins  there 
were  found  49  catbirds,  37  brown  threshers,  28  house  wrens, 
and  26  bluebirds. 

The  skins  and  feathers  of  birds  have  been  much  used  for 
ornament  by  savage  men  and  civilized  woman.  The  traffic 
in  skins  and  plumes  has  been  a  very  extensive  and  profitable 
one,  and  is  calculated  to  hasten  the  extermination  of  some 
species.  Laws  are  gradually  becoming  more  prohibitive  and 
public  sentiment  is  coming  to  support  them.  Shooting  birds 
for  sport  and  encouraging  killing  them  for  their  plumage  are 
alike  barbarous.  The  national  government,  the  states,  and 
private  individuals  should  be  encouraged  to  establish  havens 
where  birds  may  not  be  killed. 

Because  the  undigested  material  from  the  digestive  tract  and 
the  nitrogenous  excretion  from  the  kidneys  are  eliminated  to- 
gether the  manure  of  birds  is  very  rich.  Large  deposits  of  this 
material,  known  as  guano,  are  found  along  the  dry  coasts  and 
islands  on  the  west  shore  of  South  America.  It  is  deposited  by 
sea-birds  that  have  lodged  there  for  ages.  The  richest  of  these 
deposits  are  already  exhausted. 

Of  a  real  vital  value  to  man  are  the  song  and  color  and  the 
poetry  of  the  life  of  birds.  These  esthetic  features  of  birds  and 
their  wonderful  habits  and  instincts  have  attracted  thousands 
of  people  to  the  fields  and  woods.  Man  cannot  get  back  to 
nature  and  the  wild  life  in  an  open-minded  and  appreciative 
way  without  being  the  gainer.  It  is  by  no  means  a  shallow 
appeal  that  we  encourage  the  bird  life  in  order  that  nature  may 
be  kept  as  interesting  and  beautiful  as  possible.  The  fact 


436  ZOOLOGY 

that  birds  are  useful  need  not  obscure  the  fact  that  they  are 
interesting. 

444.  Special  Topics  for  Investigation  in  Field  and  Library. 

1.  Enumerate  the  special  structural  features  which  seem  to  fit  birds  for  suc- 
cessful flight.     Compare  different  birds  as  to  these  features?     What  are  the 
different  modes  of  flight?     Compare  the  flight  of  the  buzzard,  the  woodpecker, 
the  quail.     What  is  the  action  of  the  wings  in  flying?     Of  the  tail?     What  is  the 
effect  of  clipping  one  wing?     Why?     The  rate  of  flight  in  different  species  of  birds. 

2.  Study  the  group  of  birds  from  the  point  of  view  of  their  social  and  gregarious 
instincts.     Are  any  solitary?     Do  any  have  varying  social  habits  during  different 
seasons? 

3.  Make  a  general  study  of  the  migrations  of  birds,  collecting  the  facts  as  to 
range,  time,  supposed  causes,  the  effects  on  the  species  and  its  geographical  distri- 
bution, the  degree  of  exactness  in  routes  and  the  place  of  return. 

4.  Make  a  special  study  of  the  birds  of  the  locality  in  which  you  are.     Are 
there  permanent   residents?     Summer  residents?     Winter   residents?     Migrants 
(those  which  stop  only  for  a  short  time  in  the  spring  or  autumn  as  they  pass  from 
south  to  north  or  the  reverse)?     Keep  a  record  from  year  to  year  of  the  earliest 
dates  at  which  migrating  species  are  seen  in  your  locality. 

5.  What  diversity  is  there  in  the  mating  habits  of  birds?     Are  any  mono- 
gamous?    Polygamous?     What  are  the  mating  habits  of  the  cuckoo? 

6.  Make  a  report  as  to  the  nest-building  habits  of  selected  species  of  birds. 
How  do  the  nests  differ  in  location,  in  mode  of  formation,  in  perfection?     Is  there 
any  relation  between  the  character  of  the  nest  and  the  degree  of  development  of 
the  young  when  hatched?     What  range  of  variation  in  the  number  of  eggs?     In 
the  mode  of  incubation?     The  period  of  incubation?     Care  of  the  young  after 
hatching  ? 

7.  Compare  the  vocal  powers  of  birds  with  that  of  vertebrates  hitherto  studied. 
Compare  various  types  of  birds  as  to  the  range  and  character  of  their  notes.     How 
are  the  notes  of  birds  related  to  their  states  of  mind  ?     Which  are  more  vocal,  the 
males  or  the  females  ?     What  explanations  are  offered  for  this  ? 

8.  What  is  the  history  of  the  English  sparrow  in  this  country?     What  are  its 
habits?     How  do  you  account  for  its  rapid  spread? 

9.  Make  a  special  study  of  the  local  distribution  of  the  species  of  birds  known 
to  occur  in  your  vicinity.     Which  prefer  the  meadows?     The  marshes?     The 
streams?     The  woodlands?     Do  the  different  species  nest  in  the  same  regions  in 
which  they  feed  ? 

10.  The  relation  of  selected  species  of  birds  to  man.     Are  they  helpful  or  harm- 
ful to  his  interests?     Has  he  been  helpful  or  harmful  to  them? 


CHAPTER  XXIV 
CLASS  V.— MAMMALIA  (MAMMALS) 

445.  Laboratory  and  Field  Work. — Almost  any  of  the 
smaller  mammals  may  be  used  in  the  following  exercise.  Dif- 
ferent species  may  be  taken  with  profit  by  the  various  mem- 
bers of  a  class.  The  chief  points  to  be  emphasized  are  the 
habits,  instincts  and  external  structure. 

I.  Habits  and  Instincts. 

What  are  its  natural  haunts?     What  explanation  can  you 

offer  therefor? 
How  does  it  protect  itself  from  its  enemies?     What  are  its 

enemies  ?     Is  it  active  at  night  or  by  day  ?     Reasons  ? 
What  are  its  habits  as  regards  food?     Evidences? 
What  can  you  say  of  its  power  and  manner  of  locomotion  ? 

Does  the  manner  of  motion  differ  materially,  with  difference 

of  rate  ? 
Social  habits?     Mating  habits?     Care  of  young  and  their 

condition  at  birth  ? 

Is  it  scarce  or  abundant  ?     Apparent  reasons  ? 
What  are  its  relations  to  human  interests  ? 

II.  General  Form  and  Structure. 

Identify  the  regions  of  the  body  and  compare  the  condition 
found  here  with  that  seen  in  the  birds.  Relation  of  axis 
of  body  to  appendages.  Compare  the  anterior  and  posterior 
appendages  at  all  possible  points,  and  indicate  to  what  ex- 
tent the  work  done  by  each  is  indicated  by  the  structure.. 
Examine  the  claws  and  the  soles  of  the  feet. 

Examine  the  body-covering,  and  compare  the  various  parts 
as  to  color,  character  of  hair,  etc.  Does  the  hair  com- 
pletely cover  the  body?  What  is  the  position  and  use  of 
"whiskers"  ?  Evidences  for  your  conclusion. 

Locate  all  the  external  openings.  Study  the  mouth  with  its- 
contained  structures;  the  eyes:  position,  color,  lids  (is» 

437 


438  ZOOLOG\ 

there  a  nictitating  membrane  ?) ;  ears.     To  what  extent  is 
the  external  ear  developed  ? 
III.  Internal  Structures. 

(The  rat,  rabbit,  or  cat  will  serve  if  it  is  desired  to  dissect  a 
mammal).  The  student  should  be  encouraged  to  map  out 
his  own  laboratory  outline  by  reference  to  descriptive  texts. 
Two  fundamental  questions  should  be  kept  in  mind  in  all 
such  work:  How  do  the  discovered  structures  compare 
with  those  studied  in  other  animals  ?  What  service  do  they 
render  the  animal  and  how  well  are  they  adapted  to  do  the 
work  put  upon  them  ? 

446.  The  Mammalia  embrace,  on  the  whole,  the  most  highly 
developed  vertebrates.     To  this  group  man  belongs.     The  birds 
are  more  highly  specialized  in  some  respects,  but  the  mammals 
surpass  the  birds  in  the  size  and  convolutions  of  the  brain,  and 
in  the  closer  relations  between  the  mother  and  offspring  both 
before  and  after  birth.     The  form  of  parental  care  seen  in  the 
Mammalia  is  an  adaptation  resulting  in  great  advantage  to  the 
young,   and  has  also  produced  a  great  improvement  in  the 
mental  qualities  of  the  parents.     The  class  contains  forms  of 
very  varying  appearance  and  perfection  of  development,  and 
suited  to  almost  every  mode  of  life.     Many  are  aquatic,  in- 
cluding the  largest  living  animals,  the  whales;  some  burrow  in 
the  soil,  as  the  mole  and  many  rodents;  some  live  largely  in 
trees,  as  the  monkeys,  squirrels,  sloths,  etc.;  a  very  few,    as 
the  bats,  have  acquired  the  power  of  flight;  others — the  vast 
majority — live  on  the  dry  land. 

447.  General  Characteristics  of  Mammals. 

1.  Mammalia  are  air-breathing  vertebrates  in  which  the 
covering  developed  by  the  epidermis  is  hair. 

2.  In  the  female,  mammary  glands  occur  in  the  skin,  by  the 
secretions  of  which  the  young  are  nourished. 

3.  The  diaphragm,  a  muscular  partition,  completely  sepa- 
rates the  body  cavity  into  two, — an  anterior  or  thoracic  and  an 
abdominal. 

4.  With  a  few  exceptions  the  Mammalia  are  quadrupeds. 

5.  Heart  is  four-chambered;  the  temperature  of  the  blood 


MAMMALIA  439 

not  determined  by  that  of  the  surrounding  medium;  red  blood 
corpuscles  not  nucleated;  one  (the  left)  aortic  arch  persists. 

6.  Two  occipital  condyles. 

7.  Chiefly  viviparous  (Monotremes  are  oviparous);  foetus 
nourished  during  early  development  in  the  uterus  of  the  mother, 
often  being  closely  connected  therewith  by  a  complex  structure 
known  as  the  placenta. 

448.  General    Survey. — There    are    three    subclasses    of 
mammals  which  differ  in  mode  of  reproduction  and  in  degree 
of  development. 

1.  The  Monotremata  are  the  lowest  and  are  characterized 
by  the  fact  that  they  lay  eggs,  like  reptiles  and  birds;  there  is  a 
cloaca  into  which  the  alimentary,  urinary,  and  genital  canals 
open;  the  milk  glands  are  poorly  developed.     The  class  is  rep- 
resented by  the  duck-mole, — an  aquatic  form,  and  the  spiny 
ant-eater, — both  natives  of  Australia  and  neighboring  islands 
(Fig.  238). 

2.  The   Marsupialia    (pouched)    possess    a   marsupium   or 
pouch,  a  fold  of  the  skin  into  which  the  prematurely  born  young 
are  placed  and  nourished  until  able  to  take  care  of  themselves. 
The  period  of  gestation  is  short  and  the  connection  between  the 
embryo  and  the  wall  of  the  uterus  is  slight.     In  the  group  are 
embraced  the  kangaroo  and  other  Australian  forms,  and  the 
opossums  of  America.     It  is  an  interesting  fact  that  the  native 
Australasian  mammalia  all  belong  to  these  two  lower  classes 
(see  Figs.  52,  62). 

3.  In  the  Placentalia  (placenta)  there  is  a  placenta  or  mass 
of  closely  interwoven  maternal  and  embryonic  tissue  which 
unites  the  foetus  with  the  wall  of  the  uterus,  by  which  arrange- 
ment the  young  gets  its  food  and  oxygen  from  the  blood  of  the 
mother.     The  young  are  retained  much  longer  in  the  uterus,  and 
are   consequently   much   more   mature  when  born.     All    the 

.common  mammals  belong  to  this  group,  which  is  distributed 
over  the  habitable  part  of  the  earth. 

449.  Form. — The  axis  of  the  body  is  usually  separable  into 
head,  neck,  trunk,  and  tail, — though  the  last  may  be  reduced 
to  a  very  small  number  of  segments.     The  proportions  of  these 


440 


ZOOLOGY 


parts  of  course  differ  much  and  are  to  be  connected  with  the 
habits  of  life.  Most  of  the  Mammalia  are  quadrupeds  (except 
the  allies  of  the  whales,  the  porpoises,  and  the  sea-cows);  and 
all  except  man,  and  some  of  apes  most  like  him,  have  the  axis 
of  the  body  in  a  horizontal  position  supported  by  all  four 
appendages,  or  by  the  medium.  The  aquatic  forms — whales, 
porpoises,  etc.,  become  more  or  less  fish-like  in  form,  in  adapta- 

FIG.  233. 


--  n 


FIG.  233.     Diagram  of  Skin  in  Mammals,  by  Folsom. 

tion  to  the  medium.     There  is  an  enormous  range  in  size  in 
the  group, — from  the  mice  to  elephants  and  whales. 

450.  Supplementary  Topics  for  Laboratory  and  Field  Work. — Compare  the 
relative  size,  length,  etc.,  of  head,  neck,  trunk  and  tail  of  various  types  of  mammals, 
—using  well  known  animals  and  the  figures  and  descriptions  of  less  familiar  ones. 
Can  you  find  any  signs  of  connection  between  any  of  these  facts  and  known  habits 
of  the  animals  studied  ? 


MAMMALIA  44! 

Compare  the  anterior  with  the  posterior  appendages  in  a  selected  series  of 
mammals,  keeping  in  mind  the  following  points:  size,  length,  strength,  uses;  the 
number  and  character  of  the  digits.  Compare  similarly  the  corresponding  (i.  e.t 
anterior  with  anterior)  appendages  in  another  series.  Keep  in  mind,  throughout, 
the  adaptations  of  structure  to  the  conditions  of  life,  method  of  locomotion,  etc. 

451.  Integument. — The  skin,  as  in  forms  already  described, 
consists  of  two  portions, — an  ectodermal  portion,  the  epidermis, 
and  the  dermis  or  true  skin  which  is  derived  from  the  mesoderm 
(Fig.  233).  Hair  is  found  in  the  young  of  all  mammals,  though 
it  may  be  wanting  in  the  adult  (as  in  whales) ,  or  may  occur  only 
sparsely.  Hair  is  produced  by  the  epidermis,  but  is  nourished 
by  a  papilla  of  dermal  tissue  (Fig.  233,  /).  Each  hair  consists 
of  a  central  part,  or  pith,  surrounded  by  a  denser  cornified  por- 
tion, the  cortex.  Hair  differs  much  in  color  and  in  structure, — 
from  the  soft  fur  of  the  seal  to  the  quills  of  the  hedgehog  and 
porcupine.  Hair  is  considered  as  morphologically  similar  to  the 
feathers  of  birds  and  the  scales  of  reptiles. 

To  be  considered  in  the  same  connection  with  hair  are  the 
nails,  claws,  and  hoofs,  the  scales  on  the  tail  of  the  rat  or  beaver 
and  on  the  body  of  scaly  ant-eaters,  and  the  horny  material  *of 
horns  of  cattle  and  of  rhinoceros.  The  whalebone  of  whale  is 
also  an  epidermal  product  in  the  roof  of  the  mouth. 

452.  Supplementary  Studies  for  Field  and  Library. — What  is  the  economic 
value  of  the  skins  of  mammals?  How  are  they  prepared  for  the  uses  to  which  they 
are  put?  What  animals  are  prized  for  their  hairy  products  (fur,  wool,  etc.)? 
What  special  qualities  must  the  hair  have  to  be  useful  in  making  cloth? 

What  instances  can  you  adduce  of  advantageous  coloring  in  the  hair  of  mam- 
mals? What  variations  of  color  may  be  found  within  a  single  species?  What 
changes  of  color  are  possible  to  a  single  individual?  How  are  these  changes 
brought  about?  What  peculiar  qualities  have  the  quills  of  the  porcupine? 

453.  Integumentary  Glands,  derived  from  the  epidermis, 
are  common  in  mammals.  Associated  with  the  hairs  are  the 
oil  glands.  Over  various  parts  of  the  body  are  long  tubular 
sweat  glands  buried  in  the  dermis.  There  are  also  integumen- 
tary glands  producing  characteristic  odorous  substances.  These 
may  be  for  recognition  within  the  species,  or  for  protection. 
The  tear  glands  of  the  eye  are  modified  cutaneous  glands. 

The  mammary  glands,  which  are  characteristic  of  the  group, 
are  specially  developed  skin  glands,  apparently  more  allied  to 


442  ZOOLOGY 

the  oil  glands  in  their  secretion.  They  are  much  lobed,  and 
usually  have  teats  or  nipples ;  but  in  the  monotremes  these  are 
wanting,  and  the  young  merely  lick  the  secretion  from  a  "milk 
area."  The  glands  may  be  distributed  along  the  entire  ab- 
dominal surface  (carnivora)  or  confined  either  to  the  anterior 
(primates)  or  posterior  portion  (ruminants).  The  number  of 
the  milk  glands  is  correlated  in  a  general  way  with  the  number 
of  young  produced  at  a  birth. 

454.  Skeleton. — Some  of  the  more  elementary  facts  concerning  the  skeleton 
may  be  summarized  as  follows.  The  vertebrae  unite  by  flat  faces,  and  the  five  re- 
gions of  the  vertebral  column  (see  §349)  have  a  fair  degree  of  constancy  as  to 
numbers.  The  neck,  with  a  very  few  exceptions,  has  seven  vertebrae,  the  length 
of  the  neck  depending  on  the  length  of  the  vertebrae  and  not  on  their  number.  The 
trunk  vertebrae,  made  up  of  the  thoracic  and  lumbar,  usually  vary  within  the  limits 
19-23.  The  caudal  vertebrae  are  most  variable  of  all.  The  bones  of  the  skull  in 
the  adult  have  their  edges  closely  united  by  means  of  sutures  (a  species  of  close 
joint,  which  does  not  allow  of  motion).  The  lower  jaw,  the  hyoid  bone,  and  the 
small  bones  of  the  ear  are  the  only  movable  bones  in  the  mammalian  skull.  The 
lower  jaw  articulates  directly  with  the  cranium.  The  quadrate,  which  in  reptiles 
and  birds  serves  to  articulate  the  jaw  with  the  cranium,  has  apparently  changed 
its  position  and  given  rise  to  one  of  the  small  bones  of  the  middle  ear. 

The  pectoral  girdle  and  arm  bones  are  always  present,  but  in  the  whales  and 
sea-cows  the  posterior  are  lacking.  The  digits  are  typically  five  in  number.  In 
many  carnivores  these  may  be  reduced  to  four,  terminating  in  claws.  In  the  hoofed 
form3  the  toes  are  often  reduced  to  four,  three,  two,  or  even  one  (the  horse).  In 
such  cases  rudiments  of  the  remaining  digits  may  occur  in  the  form  of  splints. 

455.  Teeth. — The  teeth  are  produced  by  the  skin,  and  come 
to  be  lodged  in  pits  in  the  bones  of  the  jaws.  While  differing 
in  shape,  the  teeth  always  possess  the  crown,  the  fang  or  root, 
the  neck  and  the  pulp-cavity.  The  bulk  of  the  tooth  is  dentine 
deposited  by  the  dermis.  Over  this  is  a  layer  of  enamel  formed 
by  the  epidermis.  The  teeth  differ  from  horny  outgrowths  in 
that  both  layers  of  the  skin  are  involved.  In  this  respect  they  are 
like  the  scales  of  fishes  and  the  plates  of  the  armadillo 

The  cavity  is  more  or  less  filled  with  "pulp  tissue"  which  is 
supplied  with  nerves  and  blood  vessels.  Most  mammals  have 
only  two  sets  of  teeth, — a  milk  set  which  appears  early  and  is 
lost  and  a  permanent  set  which  replaces  the  former.  In  some 
cases,  however,  there  is  only  one  set,  and  in  a  few  (e.g.,  whales) 
no  teeth  appear  above  the  surface  of  the  gums. 

In  the  porpoises,  dolphins  and  similar  forms  the  teeth  are 


MAMMALIA  443 

numerous,  simple,  and  very  much  alike,  but  in  the  majority 
of  mammals  there  are  at  least  three  types  of  teeth.  In  the  front 
of  the  upper  jaw  (on  the  premaxillary  bones)  are  simple,  chisel- 
shaped  teeth,  the  incisors;  behind  these  (the  first  tooth  on  the 
maxilla)  is  the  canine  tooth,  usually  pointed  and  adapted  for 
tearing;  posterior  to  the  canines  are  the  grinders  or  molars. 
Those  grinders  which  replace  milk  teeth  are  sometimes  called 
premolars.  The  true  molars  do  not  have  any  representatives 
in  the  milk  set.  The  corresponding  teeth  in  the  lower  jaw  are 
similarly  named.  The  typical  number  of  teeth  is  forty-four, 
eleven  in  each  half -jaw.  This  may  be  shown  by  a  formula  in 
which  the  numerator  indicates  the  number  of  each  kind  in  one- 
half  of  the  upper  jaw  and  the  denominator  a  similar  portion 

of  the  lower:  i.    ,  c.  -,  p.  -,  m.    ,  =  44.     This  means  that  there 

3*43 
are  three  incisors,  one  canine,  four  premolars,  and  three  molars 

in  each  half  jaw,  both  above  and  below.     The  dental  formula 

2X2  3 

for  adult  man  is:  i.    ,  c.    ,  p.    ,  m.    ,  =  32.    This  may  be  written 

2  I  2  "3 

more  simply  '—.  The  numbers  are  not  always  the  same  in 
the  upper  and  lower  jaw  of  mammals. 

456.  Supplementary  Studies. — Let  the  student  determine  by  examination, 
and  write  the  dental  formulae  of  the  cat,  dog,  horse,  cow;  milk  set  in  man. 

Compare  the  molars  of  some  carnivorous  animals  with  those  of  some  herbivo- 
rous; similarly  the  canines.  Describe  the  action  of  the  jaws  in  the  act  of  chewing  in 
the  dog,  cow,  rabbit,  horse. 

457.  The  Digestive  Organs  present  the  same  regions  and 
general  arrangement  found  in  the  typical  vertebrates.  There 
are  usually  fleshy  and  movable  lips  covering  the  teeth.  Some- 
times these  are  much  extended  and  in  connection  with  the  nose 
may  become  important  organs  (snout,  proboscis)  for  the  capture 
of  food.  The  stomach  varies  widely  but  is  ordinarily  a  simple 
sac  with  muscular  walls.  Sometimes  it  is  partly  separated  into 
chambers  by  folds  (Figs.  234,  235).  This  reaches  its  greatest 
complexity  in  the  ruminants,  in  which  four  chambers  occur 
(Fig.  235).  One  of  these — the  rumen — becomes  a  temporary 
receptacle  for  the  food  which  is  first  swallowed  without  being 


444 


ZOOLOGY 


chewed.  This  peculiar  structure  is  correlated  with  the  habit 
of  rapid  feeding  and  retirement  to  less  dangerous  or  exposed 
locations,  where  the-  food  is  forced  back  to  the  mouth  in  appro- 


B 


FIG.  234.     Diagram  of  stomach  of  dog  (A)  and  rat  (B).     After  Wiedersheim. 


FIG.  235.     Diagram  of  stomach  of  Ruminant.     After  Wiedersheim. 

Questions  on  the  figure. — What  is  the  significance  of  the  term  ruminant?  Of 
what  conceivable  advantage  is  this  form  of  stomach?  What  animals  belong  to  the 
class? 

priate  quantities  and  chewed  at  leisure.  When  swallowed  the 
second  time  the  food  passes  on  to  the  glandular  divisions  of  the 
stomach. 

The  liver  and  the  pancreas  pour  their  secretions  into  the 


MAMMALIA 


445 


small  intestine  near  its  anterior  end.  The  small  intestine  is 
very  much  shorter  in  flesh-eating  animals  than  in  the  vegetable 
feeders.  At  the  junction  of  the  small  and  large  intestine  there 
is  a  blind  pouch  or  sac  (c&cum,  vermiform  appendix)  which 
is  large  in  the  Herbivora,  but  in  man  it  is  a  mere  rudiment.  It 
is  doubtful  whether  it  has  any  function  in  the  human  race. 
It  is  often  the  seat  of  serious  inflammation. 


FIG.  236. 


FIG.  236.  Diagram  of  the  heart  and  chief  vessels  in  the  mammals,  ao.,  aorta;  a./.,  left  auricle; 
o.r.,  right  auricle;  c,  carotid  artery;  d.a.,  dorsal  artery;  p.a.,  pulmonary  artery;  p.v.,  pulmonary 
vein;  s,  subclavian  artery;  v.c.,  venae  cavae  (pre-caval  and  post-caval);  v.l.t  left  ventricle;  v.r.,  right 
ventricle. 

Questions  on  the  figure. — What  kind  of  vessels  communicate  with  the  auricles? 
What  with  the  ventricles?  What  is  the  position  of  the  valves?  Trace  the  direc- 
tion of  the  blood  flow  in  the  various  parts  of  the  blood  vessels  figured.  What  is 
the  distribution  of  the  veins  and  arteries  shown  here,  i.e.,  to  what  organs  do  their 
minuter  branches  go? 

458.  Circulatory  System. — Mammals  are  warm  blooded,  but  with  lower  tem- 
perature than  is  found  among  the  birds.  It  ranges  from  35°  to  40°  C.  The  heart 
is  completely  four-chambered  as  in  birds,  the  left  side  containing  pure  blood  and 
the  right  impure  (Fig.  236).  The  aorta,  arising  from  the  left  ventricle,  has  only 
one  arch — the  left,  whereas  only  the  right  is  found  in  birds.  The  general  compari- 
son of  the  conditions  in  vertebrates  may  be  seen  from  the  table  on  page  343.  There 
is  an  hepatic-portal,  but  no  renal-portal,  circulation. 

The  lymphatic  vessels  are  an  important  part  of  the  circulatory  apparatus  in 
all  vertebrates.  Under  the  pressure  that  exists  in  the  arteries,  some  of  the  fluid 
portion  of  the  blood  finds  its  way  through  the  walls  of  the  capillaries  into  the  spaces 
among  the  tissues.  This  cannot  get  back  into  the  veins,  and  hence  it  is  desirable 
that  special  vessels  be  provided  to  get  it  back  into  the  circulation.  Starting  with 


446  ZOOLOGY 

the  irregular  spaces  in  the  tissues,  in  which  the  lymph  collects,  we  find  vessels  less 
regular  than  the  veins,  often  running  together  and  then  rebranching,  gradually 
approaching  the  body  cavity.  On  their  route  they  pass  through  knots  of  special 
tissue — lymphatic  glands,  where  colorless  amoeboid  cells  are  added.  Special 
lymphatics — the  lacteals — gather  food  from  the  intestines  and,  uniting  with  the 
general  lymphatics,  finally  empty  into  the  large  veins  in  the  neck  region.  The 
escaped  lymph  is  thus  returned  to  the  blood. 

459.  The  Respiratory  Structures  differ  from  those  of  birds 
chiefly  in  the  fact  that  they  are  confined  to  the  anterior  or 
thoracic  cavity,  in  which  they  hang  freely,  suspended  by  the 
bronchi.     There  are  no  air-sacs  outside  the  lungs,   hence  all 
the  air  passages  terminate  in  the  alveoli,  in  the  walls  of  which 
are  the  pulmonary  capillaries.     Inspiration  and  expiration  of  air 
is  affected  by  increasing  and  decreasing  the  size  of  the  chest  cavity 
by  means  of  the  muscles  between  the  ribs  and  by  the  contraction 
of  the  muscles  of  the  diaphragm  which  is  normally  arched  for- 
ward into  the  chest.     By  its  contraction  the  viscera  are  forced 
backward  and  more  space  is  given  to  the  lung,  which  at  once 
fills  the  chest  cavity  as  the  result  of  air-pressure  on  the  inside 
of  the  lung. 

460.  Nervous  System. — The  special  feature  worthy  of  note 
in  the  nervous  system  of  mammals  is  the  large  size  of  the  brain, 
especially  of  the  cerebral  hemispheres.     In  the  higher  mammals, 
particularly,  these  become  complicated  by  folds  and  convolu- 
tions by  which  the  surface  or  cortex  of  the  brain  is  much  in- 
creased.    The  brain  cells,  or  gray  matter  of  the  brain,  are  espe- 
cially abundant  in  the  superficial  part,  and  therefore  this  in- 
crease of  surface  means  that  these  cells  are  increased  in  amount 
as  compared  with  other  vertebrates.     The  intelligence  of  an 
animal  is  roughly  proportional  to  the  amount  of  the  cortex. 
The  cortex  is  a  thin  layer,  varying  from  one  to  five  millimeters 
in  thickness.     It  has  been  estimated  that  there  are  about  9,000,- 
000,000  nerve  cells  in  the  cortex  of  a  human  brain.     These  nerve 
cells  represent  about  one  five- thousandth  of  man's  total  weight, 
and  yet  they  furnish  the  basis  for  the  conscious  life  and  the  con- 
trol of  the  body.     The  fibrous  tracts  connecting  the  various  por- 
tions of  the  cortex  are  likewise  more  perfectly  developed  among 
the  mammals,  and  most  of  all  in  man. 


MAMMALIA  447 

The  organs  of  special  sense  are  similar  to  those  in  the  birds. 
The  ear  becomes  more  complicated.  There  is  usually  a  well- 
developed  external  ear,  or  pinna,  in  the  terrestrial  forms,  which 
is  often  movable  and  serves  to  gather  the  sound  waves.  The 
membranous  labyrinth  of  the  internal  ear  becomes  more  com- 
plicated than  in  any  of  the  lower  forms.  This  is  especially 
true  of  the  cochlea,  which  becomes  spirally  coiled.  The  middle 
ear  is  bridged  by  a  series  of  three  bones,  instead  of  one  or  two 
as  in  the  lower  groups  of  vertebrates  where  such  connection 
exists  at  all. 

461.  The  Urinogenital  Organs. — As  in  the  other  vertebrates  there  is  close 
connection  between  the  excretory  and  reproductive  organs  in  mammals.  The  bean- 
shaped  kidneys  communicate  by  ureters  with  a  median  urinary  bladder,  which  in 
turn  has  the  urethra  leading  to  the  outside.  The  urethra  also  serves  as  the  outlet 
for  the  sperm  in  the  male.  The  testes,  which  in  other  vertebrates  lie  in  the  body 
cavity,  pass  backward  and  descend  into  a  fold  of  the  skin,  in  the  majority  of  mam- 
mals. In  the  female,  the  ovaries  are  in  the  abdominal  cavity,  and  when  the  ova 
are  ripe  they  break  forth  into  the  cavity  and  pass  into  the  fringed,  funnel-shaped 
mouth  of  one  of  the  two  oviducts.  The  oviducts  may  be  completely  distinct, 
opening  separately  into  the  vagina  (as  in  most  rodents),  in  which  case  each  has  a 
special  portion  in  which  the  young  are  retained  during  early  development  (uterus') ; 
or  there  may  be  found  various  degrees  of  union  of  the  uterine  portions  until  there 
is  a  single  uterus  into  which  the  two  oviducts  empty  (as  in  man). 

462.  Reproduction  and  Development. — All  the  mammals 
except  the  monotremes  are  viviparous.  Impregnation  may  take 
place  in  the  oviduct  or  in  the  uterus.  In  the  Placentalia  the 
ova  are  small  and  have  little  yolk,  whereas  in  the  Monotremes 
there  is  much  yolk,  as  among  the  birds.  The  segmentation  in 
the  placental  mammals  is  complete  but  not  necessarily  equaL 
A  solid  sphere  of  cells  is  formed  which  becomes  differentiated 
into  an  outer  enclosing  layer  (thetrophoblast,  Fig.  237)  and  an  inner 
mass  of  cells  (Fig.  237,  ent.).  It  is  the  mass  of  cells  that 
gives  rise  to  the  embryonic  layers,  from  which  are  produced 
the  adult  organs.  The  trophoblast  has  little  or  no  part  in 
the  formation  of  the  embryo  proper,  but  has  a  part  in  forming 
the  foetal  membranes  so  important  in  the  group.  The  steps  of 
embryonic  development,  while  similar  in  general  to  those  de- 
scribed for  the  other  Vertebrata,  are  modified  by  the  absence  of  the 
yolk  and  the  retention  of  the  developing  egg  in  the  body  of  the 
parent.  The  embryonic  membranes — amnion  and  allantois— 


448 


ZOOLOGY 


occur  as  among  birds,  but  their  fate  is  somewhat  different.  -  The 
allantois  typically  fuses  with  the  outer  layer  of  the  amnion  (false 
amnion  Fig.  208,  am?)  and  the  trophoblast  (see  above),  and  this 
combined  tissue,  the  chorion,  becomes  connected  with  the  wall 
of  the  uterus  by  outgrowths  or  villi.  These  become  closely  asso- 
ciated with  the  tissues  of  the  mother.  This  combination  of 
maternal  and  embryonic  tissues  is  called  the  placenta,  and  is  the 
characteristic  organ  of  the  Placentalia  or  true  mammals. 

It  is  by  means  of  these  united  tissues  that  food  and  oxygen 
pass  from  the  blood  of  the  mother  into  the  blood  of  the  embryo. 
In  the  marsupials  the  attachment  is  very  slight,  and  for  this 


...  t 


in. 


FIG.  237.  Diagram  of  Segmentation  of  ovum  in  Mammals.  A,  ovum;  B,  showing  the  early 
differentiation  into  an  outer  layer  which  produces  the  trophoblast  (see  p.  447),  and  an  inner  mass 
which  produces  the  embryo;  C,  a  later  stage,  ect.,  ectodermal  portion  of  embryo;  ent.%  cells  destined 
to  produce  entoderm;  in.,  inner  mass  of  the  cells  which  form  the  embryo;  o,  outer  layer  which 
forms  t,  the  trophoblast. 

Questions  on  the  figures. — How  does  this  differ  from  the  segmentation  in  the 
•sea-urchin?  What  is  the  fate  of  the  trophoblast?  Examine  reference  texts  and 
learn  how  the  ectoderm,  entoderm,  and  mesoderm  of  the  real  embryo  (the  inner 
mass  of  cells)  are  formed. 

reason  uterine  nutrition  becomes  insufficient  relatively  early 
and  the  young  must  be  provided  for  in  some  other  way.  The 
marsupium,  in  which  the  milk  glands  open,  presents  the  solution 
of  the  problem  of  later  development  of  the  fcetus.  So  at  birth 
the  immature  young  of  marsupials  are  placed  by  the  mother  in 
the  pouch.  It  is  important  to  remember  that  the.blood  vessels 
of  the  mother  and  the  embryo  are  not  continuous.  The  blood 
of  the  embryo  is  developed  in  the  same  manner  as  its  other  tissues 
and  is  not  derived  from  the  mother  directly.  The  two  blood 


MAMMALIA  449 

currents  interchange  materials  by  osmosis  through  the  capillary 
walls.  There  is  no  direct  continuation  of  nervous  tissue  across 
from  the  mother  to  the  young.  Notwithstanding  this  lack  of 
direct  connection- each  exerts  a  very  profound  effect  upon  the 
other.  It  is  known  that  the  embryo  secretes  materials  which 
passing  into  the  blood  of  the  mother  produce  the  growth  of  the 
mammary  glands  that  leads  finally  to  lactation.  Proper  nutri- 
tion, malnutrition,  and  poisons  in  the  blood  of  the  mother 
greatly  influence  the  growth  and  normality  of  the  offspring. 

463.  Classification  of  Mammals. — In  the  introductory  sur- 
vey in  §448  the  three  subclasses  have  already  been  outlined. 
Subclass  I.  Ornitkodelphia  (bird  uterus),  or  Monotremata. — 

FIG.  238. 


FIG.  238.     Duck-bill  (Ornithorhynchus  anatinus) .     Photographed  by  Folsom. 

Questions  on  the  figure. — What  are  the  peculiarities  of  Ornithorhynchus? 
What  does  the  structure  of  its  feet  indicate  as  to  its  habits? 

Mammals  whose  mammary  glands  have  no  nipples ;  they  lay  eggs 
with  abundant  yolk,  which  are  hatched  outside  the  body,  as  in 
birds.  The  alimentary  canal  ends  in  a  cloaca.  One  ovary  is 
sometimes  incompletely  developed  as  in  birds,  and  the  oviducts 
open  separately  into  the  vagina.  The  duck-bill  or  duck-mole 
lives  in  water,  or  burrows  in  the  banks  of  streams  or  lakes.  It 
is  eighteen  or  twenty  inches  long  and  is  covered  with  soft  fur. 
Its  eggs  are  laid  in  its  burrows.  Echidna  or  the  spiny  ant-eater, 
lives  in  rocky  places  and  captures  ants  by  means  of  its  slender, 
ag 


450  ZOOLOGY 

sticky  tongue.  They  are  confined  to  Australasia  and  are  in- 
teresting chiefly  because  of  their  likeness  to  the  reptiles  and 
birds  (Fig.  238). 

Subclass  II.  Didelphia  (two  uteri),  or  Marsupialia. — Mam- 
mals with  nipples;  these  occur  in  the  pouch  on  the  ventral  surface 
of  the  body  in  which  the  immature  young  are  placed  at  birth. 
The  young  are  too  immature  to  suck  voluntarily  at  first,  and  milk 
is  forced  into  the  mouth  by  the  action  of  muscles  about  the 
gland.  The  pouch  is  usually  supported  by  two  bones  attached 
to  the  pubis  and  running  forward.  There  are  two  oviducts, 
two  uteri,  and  even  the  vaginas  may  be  paired  (Figs.  52,  62). 

Many  different  types  are  included  in  this  group.  Some  are 
rat-like  in  appearance,  others  similar  to  the  dog,  others  to  the 
bear.  Some  are  herbivorous,  some  carnivorous,  others  insec- 
tivorous. With  the  exception  of  the  American  opossum  fam- 
ily, the  living  species  are  native  of  Australasia.  Fossil  marsupials 
are  found  in  all  parts  of  the  world,  showing  that  they  are  an 
ancient  type  of  mammals  which  have  become  extinct  except  in 
the  places  cited.  Many  of  the  fossil  forms  were  of  gigantic  size. 
The  largest  living  species  is  the  kangaroo. 

Subclass   III.  Monodelphia    (one   uterus),  or   Placentalia- 
Mammals  in  which  the  young  are  connected  to  the  wall  of  the 
maternal  uterus  by  means  of  a  placenta  (see  §462) ;  two  oviducts; 
uteri  more  or  less  united  into  one;  vagina  single;  no  cloaca;  no 
marsupium.     The  segmentation  of  the  ovum  is  total. 

The  following  key  will  assist  the  student  to  get  a  view  of 
the  principal  orders  of  the  placental  mammals : 

Teeth  wanting,  or  without  enamel Edentata. 

Teeth  with  enamel. 

Hind  limbs  wanting. 

Front  appendages  with  elbow  joint Sirenia. 

Front  appendages  without  elbow  joint Cetacea. 

Hind  limbs  developed. 

Nails  of  the  digits  hoof -like Ungulata. 

Nails  claw-like. 

The  front  limbs  modified  to  form  wings, 

Cheiroptera. 


MAMMALIA  451 

No  wings. 

Thumbs  not  opposable. 

Incisors  and  canines  small, 

Insectivora. 
Incisors  chisel-shape  and  canines 

wanting Rodentia. 

Canines  large;  other  teeth  often  pointed, 

Carnivora. 

Thumbs  opposable Primates. 

Order  i .  Edentata  (toothless) . — Placentalia  in  which  the  teeth 
are  absent  or  imperfect,  being  destitute  of  enamel  and  true  roots. 
They  are  found  both  in  the  Old  World  and  in  the  New,  especially 
in  the  tropics  of  the  southern  hemisphere.  The  chief  repre- 
sentatives are  the  sloths  and  the  hairy  ant-eater  of  Central  and 
South  America,  the  armadillo  of  South  America  and  as  far  north 
as  Texas,  and  the  scaly  ant-eaters  of  Asia  and  Africa.  The 
sloths  are  sluggish  vegetarians  living  in  the  trees,  on  the  branches 
of  which  they  hang  or  climb,  back  downward,  by  means  of  their 
long  curved  claws.  The  ant-eaters  are  almost  wholly  devoid  of 
teeth,  but  have  narrow  extensible  tongues  which  they  project 
into  ant-holes,  capturing  the  ants  by  the  sticky  saliva.  The 
group  is  primitive  and  degenerate,  and  furnishes  a  noteworthy 
exception  to  the  statement  that  the  mammals  lack  an  external 
skeleton.  Overlapping  bony  scales,  or  plates  in  the  form  of 
rings,  may  furnish  a  complete  armor  by  means  of  which  they  are 
kept  from  extermination  in  spite  of  their  inoffensive,  sluggish 
habits.  This  is  best  seen  in  the  armadillo. 

Order  2.  Insectivora  (insect-feeders). — These  are  small  mam- 
mals with  clawed  digits,  which  feed  on  insects  and  other  small 
invertebrates.  The  brain  is  small  and  smooth.  The  incisors 
are  small.  Many  burrow,  and  have  special  adaptations  for 
such  a  life;  among  these  one  of  the  most  interesting  is  the  de- 
generation of  the  eyes.  The  moles,  shrews  and  hedgehogs  are 
the  chief  representatives.  The  moles  have  the  reputation  of 
eating  corn  and  other  planted  grains.  Their  burrows  frequently 
follow  the  rows.  Their  food,  however,  is  the  insects  that  collect 
about  the  planted  grain.  They  do  much  damage  to  turf  in 
making  their  burrows.  A  single  specimen  has  been  known  to 


452  ZOOLOGY 

burrow  twenty  yards  in  a  day.     For  brief  periods  and  in  reason- 
ably firm  soil  they  have  been  seen  to  dig  a  yard  in  ten  minutes. 

Order  3.  Sirenia. — A  small  group  of  aquatic  Placentalia, 
more  or  less  whale-like  in  form.  They  are  sluggish,  ungainly, 
vegetable  feeders.  They  have  no  posterior  appendages  and  the 
anterior  are  flipper-like,  though  capable  of  bending  at  the  elbow. 
They  live  near  the  shore  and  are  represented  by  two  living  genera, 
the  sea-cow  of  our  own  eastern  shores  (Manatee),  and  the 
Dugong  of  the  Indian  Ocean. 

Order  4.  Cetacea  (Whales,  Porpoises,  Etc.). — The  Cetacea  are 
aquatic  mammals  with  a  fish-like  body.  There  are  no  posterior 
appendages,  and  the  anterior  act  as  paddles,  being  without  joint. 
The  tail  is  horizontally  expanded  into  a  powerful  paddle,  and  a 
dorsal  fin  is  usually  present.  Teeth  are  present  in  the  embryo, 
but  may  be  lost  or  replaced  by  "whale-bone"  in  the  adult. 
The  stomach  is  chambered.  The  two  mammae  are  posterior. 
Hair  is  very  scant,  but  the  layer  of  fat  or  "blubber"  beneath 
the  skin  is  very  thick,  and  serves  as  a  non-conductor  of  heat. 

The  whales  are  the  largest  living  animals.  The  largest  of 
these,  the  Greenland  whale,  may  attain  a  length  of  seventy-five 
feet  or  more.  It  must  be  remembered  that  the  whales  are  air- 
breathers,  and  therefore  must  come  to  the  surface  to  breathe 
or  "blow."  The  Cetacea  prey  on  the  smaller  swimming  or 
floating  animals  found  in  the  ocean,  as  fish,  squid,  Crustacea, 
etc.  Whales  are  principally  sought  for  their  fat  and  baleen, 
or  whale-bone. 

There  are  two  principal  suborders:  the  toothed  whales,  and 
the  whale-bone  whales.  The  former  include  the  dolphins,  por- 
poises, grampuses,  narwhales,  sperm  whales,  and  beaked  whales. 
The  right  whales,  the  hump-backed  whale,  the  gray  whale,  the 
finback  whale,  and  the  sulphur  bottom  whale  lose  their  embry- 
onic teeth  and  produce  whalebone.  Food  is  taken  with  the 
mouth  open  and  the  water  is  forced  out  through  the  whale-bone, 
eaving  the  "catch"  in  the  mouth.  The  sulphur  bottom  may 
reach  a  length  of  ninety-five  feet  and  a  weight  of  150  tons. 

Order  5.  Ungulata  (hoofed  animals). — This  order  includes  a 
great  number  of  animals,  chiefly  herbivorous,  that  walk  on  their 
toes.  In  these  forms  the  horny  growth  which  we  have  so  fre- 


MAMMALIA  453 

quently  found  in  vertebrates  at  the  end  of  the  digits  takes  the 
form  of  a  hoof.  Toes  are  usually  not  more  than  four  in  number. 
The  canine  teeth  are  small  or  absent.  The  following  suborders 
are  important. 

Suborder  (a)  Artiodactyla  (even-toed). — These  are  ungulates 
with  toes  reduced  to  four  or  two.  The  third  and  fourth  toes 
persist  and  bear  the  weight  of  the  animal,  and  the  second  and 
fifth,  if  present,  may  or  may  not  touch  the  ground.  The 
mammae  are  distributed  along  the  entire  abdomen  (hog)  or  are 
confined  to  the  pelvic  region  (ox). 

This  is  a  splendid  group  containing  some  of  man's  most 
valuable  food  animals.  Included  in  it  are  cattle,  sheep,  goats, 
antelopes,  elk,  deer;  the  giraffes;  the  camels  and  llamas;  the 
hippopotami ;  and  the  hogs  and  peccaries.  The  ox,  sheep,  goat, 
deer,  and  giraffe  are  ruminants  (see  p.  444). 

The  chief  native  American  forms  are  llama,  elk,  moose, 
black-tailed  deer,  pronghorn  antelope,  Rocky  Mountain  goats, 
bighorn,  musk  ox,  and  bison.  The  bison  was  formerly  one  of  the 
most  abundant  of  our  American  ruminants.  Now  there  are 
only  a  small  number  of  specimens  in  captivity  and  fewer  still 
wild.  Most  of  these  forms  are  capable  of  domestication,  be- 
cause of  their  tractable  disposition  and  their  feeding  habits. 
They  are  very  valuable  both  for  their  hides  and  their  food  prod- 
ucts. In  addition  to  the  flesh  foods,  milk  and  its  products  of 
butter  and  cheese  are  among  the  most  prized  of  human  foods. 
Milk,  as  are  eggs,  is  a  complete  food ;  that  is,  it  contains  all  the 
constituents,  both  organic  and  inorganic,  necessary  to  support 
growth  and  development,  as  is  shown  by  the  fact  that  it  is  the 
sole  supply  of  food  for  the  young  immediately  after  birth. 

Suborder  (b)  Perissodactyla  (odd-toed). — These  are  character- 
ized by  the  fact  that  the  weight  of  the  body  rests  on  the  third  or 
middle  toe,  the  others  being  more  or  less  reduced.  The  stomach 
is  simple.  There  is  no  proboscis.  The  mammae  are  few  and 
confined  to  the  pelvic  region.  The  most  common  examples  are 
the  horse  and  its  allies,  in  which  the  third  is  the  only  digit,  and 
the  rhinoceros,  which  has  the  second  and  fourth,  as  well.  It 
is  known  that  the  remote  ancestors  of  the  horse  had  a  second 
and  a  fourth  toe  where  only  splints  occur  now,  and  even  a  first 


454  ZOOLOGY 

and  a  fifth,  where  now  there  is  no  trace  of  either.  The  horse 
has  been  domesticated  as  long  as  we  have  records  and  there  are 
as  many  breeds  of  horses  as  we  found  of  chickens, — some  half  a 
hundred.  The  ass  and  zebra  belong  to  the  same  genus, — Equus. 
They  both  cross  with  the  horse.  Tapirs  are  included  in  this 
suborder. 

Suborder  (c)  Proboscidea  (with  proboscis). — Two  living  and 
many  extinct  species  of  huge  Placentalia  with  five  digits,  each 
with  a  distinct  hoof.  The  nose  is  much  developed  into  a  pre- 
hensile organ,  with  corresponding  changes  in  the  skull  for  at- 
tachment of  muscles.  The  skin  is  thick  and  loose.  The  upper 
incisors  grow  enormously,  forming  the  tusks  characteristic  of 
the  group.  There  are  no  canines;  molars  are  very  complex. 
The  two  teats  are  thoracic.  The  largest  of  the  land  mammals, 
the  elephants  and  the  extinct  mastodon  and  mammoth,  belong 
here.  They  are  now  confined  to  the  tropical  regions  of  Asia 
and  Africa,  though  in  geological  times  they  seem  to  have  had  a 
world- wide  range.  The  tusks  of  species  of  elephants,  both  living 
and  extinct,  furnish  the  ivory  of  commerce. 

Order  6.  Carnivora  (flesh-eaters). — The  Garni vora  are  four- 
or  five-toed  animals  with  the  digits  ending  in  claws.  The 
canines  are  well  developed,  strong  and  curved.  The  other  teeth 
are  often  pointed  and  adapted  to  holding  or  tearing.  Muscles 
of  mastication  are  especially  well  developed.  Mammas  are 
numerous,  occurring  along  the  entire  abdomen.  There  are  two 
types  of  Carnivora — terrestrial  and  marine.  To  the  first  belong 
the  bear  family,  which  is  perhaps  the  least  specialized  group;  the 
dog  family,  including  dogs,  wolves,  foxes,  jackals;  the  cat  family, 
including  lions,  tigers,  leopards;  hyenas;  many  fur-bearing  ani- 
mals— as  otters,  weasels,  minks,  martens,  badgers,  wolverines, 
and  skunks.  There  are  more  than  forty  species  of  these  fur- 
bearing  animals  in  North  America.  The  seals  and  walruses 
belong  to  the  marine  group.  In  these  forms  the  appendages 
have  become  adapted  to  the  water  habit,  the  digits  bearing 
intervening  webs. 

The  principal  American  cats  are:  the  wild  cat  (also  called 
bob  cat  and  catamount),  Canada  lynx,  the  cougar  (also  called 
puma,  panther  and  mountain  lion),  and  the  jaguar  of  Mexico 


MAMMALIA  455 

and  South  America  which  is  the  largest.  Our  native  dogs  are 
the  timber  wolf,  the  coyote,  the  red  fox,  the  blue  fox,  the  gray 
fox.  We  have  several  species  of  bears, — polar,  black,  grizzly, 
and  cinnamon. 

The  greatest  of  the  cats  are  the  tigers,  which  may  reach  a 
length  of  ten  feet,  and  the  lions.  These  with  the  leopard  are 
Southern  Asiatic  and  African  types. 

The  order  embraces  many  very  powerful  and  intelligent 
animals  which  are  well  adapted  to  win  in  the  struggle  for  life, 
if  it  were  not  for  human  interference.  In  the  presence  of  man, 
however,  all  those  which  are  not  suited  to  domestication  are 
gradually  disappearing;  some  because  of  their  dangerous  quali- 
ties, others  because  of  the  value  of  their  products.  The  group 
is  not  used  to  any  considerable  extent  as  food. 

Order  7.  Rodentia  (gnawing  animals). — The  rodents  are 
small  mammals  with  clawed  digits.  They  have  no  canine  teeth, 

FIG.  239. 


FIG.  239.     The  Jumping  Rat  (Perodipus  richardsoni) ,  adult  male.      Photographed  from  life  by 

Dr.  R.  W.  Shufeldt. 

Questions  on  the  figure. — What  order  of  mammals  is  illustrated  by  this  form? 
What  explanations  are  offered  as  to  the  cause  of  the  light  color  of  the  belly  and  the 
dark  color  of  the  backs  of  animals?  Of  what  conceivable  advantage  is  the  differ- 
ence in  coloration?  How  does  the  tail  of  this  species  compare  with  that  of  our 
common  rat? 


456 


ZOOLOGY 


but  have  well-developed  chisel-shaped  incisors  which  continue 
to  grow  as  they  are  worn  at  the  extremity.  The  chisel  edge  is 
preserved  by  the  fact  that  the  enamel  is  chiefly  in  front,  and 

FIG.  240. 


FIG.  240.     The  Fox  Squirrel  (Sciurus  ludovicianus) .     Photographed  by  Folsom. 

FIG.  241. 


FlG.  241.     Porcupine  (Erethizon).     By  Dr.  J.  W.  Folsom. 

Questions  on  the  figure. — To  what  order  of  mammals  does  the  porcupine 
belong?     What  are  its  peculiarities  of  habit  and  structure? 

the  exposed,  softer  dentine  behind  is  worn  away  more  rapidly 
by  being  used.  The  brain  is  smooth.  The  mammae  are  ab- 
dominal. The  rodents  have  world-wide  distribution,  and  are 


MAMMALIA  457 

especially  well  represented  in  North  America.  The  principal 
types  are  the  rats  and  mice,  many  of  which  are  close  followers 
of  civilized  man;  squirrels,  chipmunks,  and  prairie  dogs, 
gophers,  beavers,  hares  and  rabbits,  and  porcupines.  The 
rodents  feed  on  vegetable  diet,  and  are  destructive  of  many 
plants  and  grains  which  man  uses  for  food.  Notwithstanding 
man's  efforts  to  destroy  them  their  remarkable  power  of  repro- 
duction enables  the  more  aggressive  families  to  hold  their  own. 


\y/ 

FIG.  242.     Flying  Fox  (Pteropus).     U.  S.  Dept.  Agriculture  Year-book,  1898. 

Questions  on  the  figure. — What  is  the  structure  and  arrangement  of  the  wings 
in  such  a  form  as  this?     To  what  order  of  mammals  does  this  type  belong? 

Rats  are  among  the  worst  of  mammalian  pests.  They  follow 
man  closely  and  are  very  destructive  of  certain  crops  and  of 
stored  grains  and  other  foods.  Beside  this  the  rats,  and  in  less 
degree  other  rodents,  are  the  carriers  of  the  bubonic  plague  and 
possibly  other  diseases.  Fleas  carry  the  disease  from  rats  to 
man. 

Order  8.  Chiroptera  (hand-winged;  the  bats). — These  are  mam- 
mals in  which  flight  is  made  possible  by  a  web  or  fold  of  the  skin 


458 


ZOOLOGY 


stretching  between  the  much  extended  fingers  of  the  anterior 
appendages;  between  the  arm,  body,  and  the  hind  legs;  and 
thence  even  to  the  tail.  The  thumb  and  posterior  digits  are 
clawed.  The  sternum  has  a  keel  as  in  birds.  The  mammae  are 
thoracic.  The  bats  are  the  only  mammals  capable  of  active 
flight.  They  feed  on  insects  or  fruits.  The  members  of  one 
family,  the  true  vampires,  live  on  blood  of  warm  blooded  animals. 
They  cut  through  the  skin  with  sharp  front  teeth  and  lick  up 
the  escaping  blood. 

Bats  are  small  animals  and  are  chiefly  nocturnal  in  habit,  at 
which  time  they  fly  actively.  During  the  day  they  seek  retired 
and  dark  places  where  they  hang  head  downward,  holding  on 
by  their  claws.  Those  in  temperate  regions  hibernate  during 
cold  weather,  often  using  caves  for  this  purpose.  (Pig.  242.) 

Order  9.  Primates  (first  or  highest). — With  the  exception  of 
man  the  primates  are  arboreal  in  habit.  In  adaptation  to  this 

FIG.  243. 


FIG.  243. — Hand  and  foot  of  Chimpanzee.     From  Home  and  Country  Magazine. 

Questions  on  the  figure. — Which  is  hand,  and  which  foot?  In  what  respects 
do  they  differ?  How  do  they  differ  from  the  hand  and  foot  of  man?  In  which 
is  the  difference  from  the  human  condition  greater?  What  is  the  functional 
meaning  of  these  differences? 

the  thumb  and  great  toe  are  usually  opposable  to  the  other 
digits,  as  in  the  human  hand.  The  digits  are  armed  with  nails 
which  are  in  some  cases  claw-like.  The  cerebrum  is  large  and 


MAMMALIA  459 

in  higher  forms  much  convoluted.  Mammae  chiefly  thoracic 
(abdominal  in  some  lower  forms).  The  group  embraces  the 
lemurs,  monkeys  of  various  kinds,  baboons  with  non-prehensile 
tails,  the  tailless  apes  most  like  man,  and  man  himself.  While 
the  primates  have  produced  some  species  with  the  highest  in- 
telligence to  be  found  among  mammals,  and  man  places  himself 
at  the  head  of  the  animal  series,  many  of  the  bodily  character- 
istics of  the  primates  are  rather  primitive.  For  example,  the 
habit  of  walking  on  the  whole  foot  (plantigrade),  the  nails,  the 
teeth,  and  many  other  structures  are  regarded  as  more  primitive 
and  less  specialized  than  corresponding  structures  in  such  groups 
as  the  carnivores  or  ungulates.  We  are  more  near  the  center 
of  the  shaft  of  evolution.  (See  Fig.  251). 
The  chief  divisions  of  the  primates  are: 

1.  Lemuridae,  or  lemurs. 

2.  New  World  monkeys,  with  broad  and  flat  noses. 

3.  Old  World  monkeys,  having  narrower  nose  with  more 
bridge. 

4.  The  man-like  apes  of  the  Old  World. 

5.  Some  intermediate  extinct   types  of  man.     (Found  in 
caves  of  Europe.) 

6.  The  Genus  Homo, — modern  man. 

The  primates  below  man  are  found  chiefly  in  tropical  regions 
and  more  abundantly  south  of  the  equator.  They  feed  largely 
on  fruit  and  insects,  though  some  eat  birds  and  other  small 
animals.  Many  of  them  are  social,  or  at  least  gregarious,  and 
their  habits  of  life  are  interesting  and  suggestive  in  a  high  degree, 
when  we  consider  their  possible  relation  to  the  human  species. 

464.  Additional  Notes  on  the  Habits  of  Mammals. — We 

have  seen  that  mammals  have  succeeded  in  occupying  in  over- 
whelming numbers  the  land,  in  much  less  degree  the  water,  and 
least  of  all  the  air.  We  have  classified  them  as  insectivorous 
(moles,  ant-eaters,  and  bats) ;  carnivorous,  as  the  beasts  of  prey; 
herbivorous,  as  the  hoofed  animals,  rodents,  and  kangaroo;  or 
omnivorous,  as  the  pigs  and  man.  They  are  very  versatile 
and  have  dominated  the  earth  since  the  tertiary  epoch  when 
they  supplanted  the  immense  reptiles  of  the  earlier  ages. 


460  ZOOLOGY 

One  of  the  most  noteworthy  facts  in  connection  with  the 
group  is  the  degree  of  care  given  to  the  young  by  the  parents, 
especially  the  mother.  This  is  true  not  merely  in  gestation  but 
after  birth  in  the  attendance  of  the  mother  to  the  needs  of  the 
young,  both  in  supplying  food  and  in  protecting  from  danger. 
It  must  be  remembered  that  this  is  done  at  the  expense  of  the 
parent's  safety.  It  means  that  the  species  may  be  kept  alive 
by  the  birth  of  a  smaller  number  of  young,  because  more  will 
reach  maturity  than  if  left  early  to  shift  for  themselves;  and, 
further,  that  a  higher  development  of  the  young  becomes  possible 
owing  to  the  increased  length  of  youth.  The  degree  of  develop- 
ment at  birth  is  quite  variable.  In  a  general  way  it  is  less  in  the 
case  of  those  whose  parents  can  best  protect  the  helpless  young. 
For  example  the  young  of  the  Carnivora  and  of  the  Primates 
are  much  less  able  to  take  care  of  themselves  at  birth  than  the 
young  of  the  Herbivora.  Many  biologists  have  called  attention 
to  the  fact  that  the  greater  care  of  the  young  implies  higher 
instincts  and  intelligence  on  the  part  of  the  parent.  Any  such 
improvement  is  subject  to  the  action  of  natural  selection  as  an 
advantageous  characteristic.  In  turn  a  longer  youth  or  period 
of  development  is  demanded  for  the  maturing  of  these  higher 
instincts,  thus  in  turn  making  a  new  demand  on  the  parent  for 
more  care  and  training. 

The  social  instinct  is  well  represented  among  mammals. 
This  may  vary  from  collection  in  mere  shoals  or  herds  where 
food  is  abundant,  to  groups  organized  for  offense  and  defense 
and  for  work, — as  wolves,  deer,  beavers,  and  men.  Indiscrimi- 
nate mating  is  the  rule,  yet  in  some  instances  strict  monogamy  is 
found.  In  many  cases  mates  are  won  by  force,  and  this  tends 
to  result  in  the  selection  and  propagation  of  the  strong.  The 
struggle  among  the  males  is  accompanied  by  the  development 
in  them  of  numerous  structures  which  the  females  do  not  pos- 
sess at  all  or  at  least  in  such  degree: — as  antlers,  horns,  tusks, 
manes, — and  greater  size. 

Mammals  do  not  have  so  great  freedom  of  motion  as  the 
birds,  and  hence  do  not  make  as  much  use  of  migration  to  escape 
winter  conditions.  They  tend  to  keep  up  an  active  life,  like  the 
rabbit  and  wolf,  or  hibernate,  like  the  woodchuck.  There  are  some 


MAMMALIA  461 

forms  however  that  do  migrate.  These  migrations  may  be  re- 
lated to  the  breeding  habits  or  to  food  supply  rather  than  to  tem- 
perature primarily.  Rats,  the  rat-like  lemmings,  fur-seal,  and 
reindeer  migrate.  Wonderful  stories  are  told  of  the  lemmings 
of  Sweden  and  their  fierce  instinctive  migration,  from  which  noth- 
ing can  turn  them,  until  they  plunge  into  the  sea  and  swim  until 
they  perish. 

The  hibernating  mammals  must  find  a  place  where  they  will 
not  quite  freeze.  All  their  vital  activities  are  lowered  and  their 
temperature  falls  gradually  until  in  some  instances  it  closely 
approaches  the  freezing  point.  Usually  this  state  must  be  ap- 
proached somewhat  gradually.  The  larger  hibernating  mam- 
mals like  the  bear  must  seek  caves  or  be  satisfied  with  a*  hollow 
log  or  a  shallow  den.  Burrowing  forms  fare  best,  as  they  merely 
dig  themselves  in  below  the  frost  line.  The  smaller  ones  must 
find  the  safer  places,  since  they  lose  their  heat  more  rapidly. 

Different  species  vary  greatly  in  the  duration  of  their  winter 
sleep.  A  few  hibernate  through  the  whole  of  the  cold  weather 
without  interruption,  as  the  woodchuck.  Some  may  hibernate 
only  in  the  coldest  weather  and  for  a  brief  time.  Others  wake 
up  from  time  to  time  and  seek  food.  Although  the  vital  proc- 
esses are  low  they  keep  going.  This  uses  up  material.  Since 
no  food  is  taken,  the  fatty  reserves  of  the  body  are  drawn  upon. 
Animals  go  into  the  sleep  fat  and  come  out  thin. 

It  is  in  the  higher  mammals  that  one  finds  the  greatest  dis- 
play of  intelligence  to  be  seen  in  the  animal  kingdom,  and  it 
is  in  man  that  intelligence  and  reason — whose  first  beginnings  in 
animals  no  one  can  mark — find  their  culmination.  That  these 
high  qualities  are  closely  correlated  with  the  great  development 
of  the  brain  there  can  be  no  doubt.  The  great  progress  of  man 
in  getting  mastery  of  the  earth  is  one  of  the  most  interesting 
aspects  of  the  same  general  problem  of  evolution  and  adapta- 
tion which  gives  unity  to  the  subject  matter  of  zoology.  Thus 
the  sciences  which  pertain  to  man  in  all  his  various  interests 
have  in  some  measure  their  foundation  in  the  science  of  zoology. 
(See  Chapter  XXV.) 

465.  Mammals  in  Relation  to  Man. — Mammals  are  not  merely 
closest  to  man  in  matter  of  kinship,  but  they  are  his  most 


462  ZOOLOGY 

valuable  allies  in  the  struggle  for  existence.  Doubtless  they  were 
also  his  worst  enemies  in  his  earlier  history.  They  furnish  him 
meat,  milk,  clothing,  labor,  transportation,  sport,  and  compan- 
ionship. They  have  always  furnished  his  favorite  game  when 
he  was  not  engaged  in  hunting  other  men.  His  domesticated 
animals  are  chiefly  from  this  class.  In  most  cases  the  beginnings 
of  domestication  go  back  to  our  earliest  records  and  we  can 
only  guess  which  of  the  species  of  wild  animals  he  began  with. 
In  these  cases  he  has  bred  and  selected  them  in  so  many  differ- 
ent ways  that  scores  of  distinct  varieties  are  now  scattered  over 
the  face  of  the  earth.  Now  that  we  are  coming  to  understand 
more  of  the  method  of  transmitting  many  of  the  unit  characters 
(see  §493)  we  shall  be  able  to  recombine  desirable  qualities 
more  systematically,  and  doubtless  new  and  desirable  breeds 
will  arise  still  more  rapidly. 

Man  has  almost  destroyed  those  fierce  mammals  that  were 
his  terror  at  the  beginning.  They  are  all  but  extinct  or  have 
migrated  to  the  jungles  of  the  tropics.  There  are,  however,  a 
number  of  smaller  ones  that  he  has  not  been  able  to  control. 
The  cat  family  is  very  much  less  a  menace  to  human  interests 
now  than  the  rodents.  Most  of  these  are  pests.  Rats  and  mice 
are  the  most  hurtful.  They  attack  all  sorts  of  vegetable  matter 
both  in  the  fields  and  in  storage.  Rats  attack  poultry.  They 
gnaw  and  nest  in  all  sorts  of  manufactured  materials.  There 
is  no  doubt  that  they  damage  hundreds  of  millions  of  dollars 
worth  of  materials  in  the  United  States  every  year. 

Rabbits  in  some  parts  of  the  world  are  scarcely  less  injurious. 
They  attack  all  kinds  of  tender  growths,  and  in  winter  may  eat 
the  bark  fnzfm  young  trees  as  high  as  they  can  reach.  These 
small  rodents  multiply  rapidly  and  have  become  notorious  pests, 
especially  when  introduced  into  new  regions  where  their  natural 
enemies  were  not  found.  Historic  instances  are,  rabbits  in 
Australia,  mongoose  in  Jamaica  and  the  Bermudas,  and  the 
rat  into  our  own  country. 

It  is  not  always  easy  to  classify  forms  as  helpful  or  hurtful. 
Forms  like  wolves,  coyotes,  foxes,  mink  and  weasels  undoubtedly 
attack  the  poultry,  sheep  and  the  larger  mammals  that  we  culti- 
vate, and  should  be  killed.  In  eighteen  months  in  1912  and  1913 


MAMMALIA  463 

in  the  state  of  Texas  alone,  were  killed  98,600  wolves  and  wild 
cats,  including  53  panthers  and  22  leopards.  And  yet  when 
the  domestic  animals  are  properly  protected,  most  of  these  forms 
live  upon  rabbits  and  other  rodents,  and  insects.  The  balance  in 
nature  is  so  complex  that  we  cannot  prophesy  all  the  results  that 
may  come  from  the  destruction  of  a  species  or  the  introduction 
of  a  new  one.  Our  most  pressing  problems  relating  to  noxious 
animals  lie  in  the  control  of  the  rodents,  the  insects,  and  the 
sporozoa. 

466.  Supplementary  Topics  for  Field  and  Library. 

1.  Enumerate  the  native  species  of  mammals  known  by  you 
to  be  found  in  your  locality,  and  determine  to  which  of  the 
orders  of  mammals  they  belong.     Are  you  impressed  that  the 
number  of  native  species  of  mammals  is  large  or  small  as  com- 
pared with  other  animals  ?     Are  the  individuals  of  these  species 
numerous  or  not  ?     How  do  you  account  for  the  facts  you  have 
discovered  ? 

2.  Enumerate  the  species  of  domestic  mammals  in  your  lo- 
cality.    Are  they  related  to  any  of  the  native  species?     Trace 
the  history  of  some  of  the  most  important  domestic  types. 
What  do  you  know  of  the  mammals  domesticated  in  other 
parts  of  the  earth? 

3.  Make  a  report  on  the  ruminants:  their  habits,  their  dis- 
tribution over  the  earth,  and  their  uses  to  man. 

4.  Make  an  investigation  of  the  breeds  of  cattle,  or  horses,  or 
goats,  or  sheep.     Find  out  some  of  the  strong  and  weak  points 
of  each  breed. 

5.  What  is  known  of  the  geological  history  of  the  horse 
family  ?     Is  the  horse  a  native  of  America  ? 

6.  What  is  the  history  of  the  introduction  of  rabbits  into 
Australia  ?     Can  you  cite  any  similar  history  of  rodents  in  this 
country  ? 

7.  Report  on  furs  and  fur-bearing  animals.     What  is  the 
present  state  of  the  seal  fisheries  of  our  Pacific  coast?    What 
steps  are  necessary  to  the  preservation  of  the  seal?     On  what 
zoological  grounds  are  these  steps  necessary? 

8.  Make  a  report  on  the  habits  and  instincts  of  the  beaver. 
Describe  the  nature  of  its  social  life. 


464  ZOOLOGY 

9.  Report  on  the  condition  of  primitive  man.  Which  of 
man's  instincts  have  been  of  most  use  to  him  in  his  develop- 
ment ?  '  What  are  the  principal  faculties  separating  him  from 
the  other  primates  ?  Do  all  men  possess  these  in  equal  degree  ? 
What  is  the  distribution  of  the  principal  races  or  varieties  of 
the  human  species?  What  are  the  chief  differences  between 
these  races  ? 


CHAPTER  XXV 

CLASS  MAMMALIA  (CONTINUED):  MAN 

467.  General    Statement. — Man    agrees    with    the    higher 
primates  in  all  the  essential  structures  and  functions,  as  we  have 
seen.     In  many  minor  details,  too,  he  resembles  them.     The 
form  of  the  fingers  and  the  flat  nails;  the  number  and  arrange- 
ment of  the  teeth;  the  shape  of  the  face,  and  numerous  other 
superficial  things  might  be  chosen  to  show  the  likeness  between 
man  and  the  higher  apes.     It  is  agreed  by  zoologists  that  there 
is  less  difference  of  structure  between  man  and  the  higher  apes 
than  between  the  apes  and  the  monkeys. 

In  general,  man  agrees  with  all  animals  in  all  that  is  necessary 
to  make  an  organism  an  animal;  he  agrees  with  all  the  verte- 
brates in  the  vertebrate  characteristics;  he  agrees  with  all  the 
mammals  in  their  distinctive  features ;  and  finally  he  agrees  with 
the  primates  in  the  qualities  that  separate  them  from  the  lower 
mammals. 

468.  The   Structural  Distinctions  Between  Man  and  the 
Other  Mammals. — Man  differs  from  the  other  primates  in  the 
fact  that  he  is  more  upright  in  position.     This  brings  about  a 
greater  development  of  the  hind  legs  and  the  pelvic  bones  for 
support  and  leaves  the  hands  free  to  serve  the  individual  in  ways 
other  than  locomotion.     He  differs  also  in  the  size  of  the  brain 
and  in  the  convolutions  of  it.     The  brain  of  the  average  man 
weighs  from  two  to  three  times  that  of  the  gorilla,  while  the 
heaviest  known  human  brain  is  only  twice  as  heavy  as  those  of 
the  lowest  races  of  men.     There  is  a  corresponding  difference 
in  the  shape  of  the  head  (compare  Figs.  244  and  245) .     The  great 
toe  cannot  be  opposed  to  the  other  toes  as  in  the  case  of  the  apes 
(Fig.  243),  or  as  man  can  oppose  his  thumb.     There  is  freer 
motion  of  the  big  toe,  however,  in  newly  born  babes  than  in 
adults.     The  canine  teeth  of  man  are  somewhat  less  developed, 
and  the  body  covering  of  hair  is  not  so  pronounced.     These 

30  465 


466 


ZOOLOGY 


.  differences,  however,  except  that  relating  to  the  brain,  are  of 
minor  importance. 

FIG.  244. 


FIG.  244. — Human  skulls. — The  one  in  heavy  outline,  with  portions  shaded,  is  a  Caucasian  skull. 
That  merely  outlined  (in  dotted  line)  is  of  an  Australian  negro. 

FlG.    245. 


FIG.  245. — Skull  of  young  orang  outang  (dotted  outline)  superimposed  on  that  of  a  young  child. 

469.  Functional  Distinction  Between  Man  and  the  Other 
Mammals. — While  the  structural  differences  are  not  sufficient 
to  remove  man  from  the  group  of  primates,  because  they  are 
relatively  insignificant  in  themselves,  the  powers  and  activities 
and  modes  of  life  that  have  arisen  from  these  do  serve  to  place 
man  clearly  into  a  group  of  his  own.  These  great  improvements 
in  powers  have  accompanied  and  are  in  some  measure  due  to  the 
gaining  of  the  upright  position  and  to  the  use  of  the  hands  as  the 


MAN 


467 


instrument  of  the  brain  and  to  the  development  of  spoken  lan- 
guage.    These  three  things — the  enlarging  brain,  the  free  hands, 

FIG.  246. 


FIG.  246. — Skull  of  adult  orang  outang. 

Questions  on  Figs.  244-246. — Enumerate  the  similarities  and  the  differences 
apparent  in  these  skulls.  In  what  respects  is  the  negro  skull  intermediate  between 
the  orang  and  the  Caucasian?  Is  the  difference  between  the  orang  and  human 
greater  in  youth  or  in  the  adult?  What  is  the  probable  significance  of  this? 
What  are  the  zig-zag  lines  in  some  of  the  figures?  What  is  the  meaning  of  the 
shaded  area  above  the  cheek-bone  and  back  of  the  eye?  How  does  the  lower 
jaw  in  the  higher  races  compare  with  that  of  the  lower?  The  meaning  of  this? 

FIG.  247. 


PIG.  247.  The  outline  of  the  brain  of  an  orang  outang.  Adopted  from  Gegenbauer  and  Vogt 
and  Gratiolet.  Front  portion.  F  to  O,  cerebrum;  C,  cerebellum;  M,  medulla  and  spinal  cord;  F, 
the  frontal  lobe;  P,  the  parietal  lobe;  O,  the  occipital  lobe;  T,  the  temporal  lobe;  R,  the  fissure  of 
Rolando;  S,  the  fissure  of  Silvius. 


468 


ZOOLOGY 


and  the  growing  language — would  tend  to  train  and  improve 
one  another.     The  brain  working  through  the  hands,  means  the 


Pic.  248. — Brain  of  Hottentot  woman.     From  Mill's  "Text-book  of  Animal  Physiology,"  Copy- 
right D.  Appleton  &  Co.     Lettering  as  in  Fig.  247. 

FIG.  249. 


FlG.  249. — Brain  of  Gauss,  the  mathematician.     From  Mill's  "Text-book  of  Animal  Physiology;' 
Copyright  D.  Appleton  &  Co.    Lettering  as  in  Fig.  247. 

Questions  on  Figs.  247-24Q.--Identify  and  compare  the  cerebrum  in  these 
three  brains.  What  are  the  chief  differences?  Compare  the  mass  in  front  of  the 
fissure  of  Silvius  (that  is  the  frontal  lobe),  in  the  three.  What  is  the  reason  of  the 
connection  between  the  numerous  convolutions  and  mental  development?  Which 
increases  more,  the  cerebrum  or  the  cerebellum,  as  we  ascend  the  scale?  The 
meaning  of  this? 


MAN  469 

making  of  implements,  of  dwellings,  and  getting  numerous  forms 
of  mastery  over  nature.  The  brain  working  through  language 
and  in  the  more  intimate  parental  relations  brought  about  by 
homes,  makes  possible  education  and  the  training  of  the  young 
in  the  discoveries  of  the  race.  The  increase  of  the  cerebral 
cortex  and  the  closer  association  of  the  different  parts  of  this 
cortex  are  in  some  way  related  to  the  better  consciousness  and 
memory  in  man.  The  use  of  language  has  been  the  great  in- 
strument whereby  the  power  of  abstract  reasoning  has  been 
developed,  and  this  has  produced  a  further  increase  in  size  and 
complexity  of  the  brain. 

470.  Mental    Life    of   Man,   Habits,  Instincts,   Reason. — 

Fundamentally,  the  mental  life  of  man,  no  less  than  that  of  the 
lower  animals,  is  the  function  of  the  brain  and  its  accompanying 
systems  of  organs.  Like  that  of  all  the  other  animals,  much  of 
man's  life  and  activities  is  made  up  of  habitual  actions  that  have 
been  acquired  through  trial  and  experience,  and  the  crystallizing 
of  these  through  memory.  The  child  takes  its  first  steps  in 
learning  very  much  as  the  lower  animals  do.  It  is  stimulated 
and  it  acts  somewhat  at  random.  If  satisfaction  follows  the 
action  it  is  repeated.  If  dissatisfaction  follows,  some  other 
action  is  tried.  Finally  it  hits  upon  a  suitable  action.  This  is. 
the  trial  and  error  method. 

Much  of  it,  quite  as  really  as  in  the  lower  animals,  is  of  those 
more  mysterious  impulses  (instincts)  that  we  do  not  get  by  our 
own  experiences,  but  which  belong  to  our  make-up  in  some  way 
as  an  inheritance  from  the  past  of  our  ancestors.  We  do  not 
realize  how  much  of  our  own  life  is  controlled  by  these  inherited 
instincts.  These  instincts  predispose  us  to  do  certain  things. 
The  mind  of  man,  working  upon  these  experiences  and  internal 
feelings,  and  using  language  to  make  clear  to  others  his  states 
and  to  convince  them  of  his  conclusions,  has  reached  what  may 
well  be  regarded  as  the  highest  human  power — that  of  abstract 
reasoning: — of  saying,  (i)  this  is  true;  and  (2)  this  is  true;  (3) 
therefore,  this  is  true.  The  study  of  this  feature  of  Zoology  is 
known  as  Psychology.  The  lower  animals  have  their  psychology 
as  well  as  man,  but  it  is  not  so  far-reaching  nor  complex. 


470  ZOOLOGY 

471.  The   Social  Instincts  and  their  Result  in  Man. — In 

the  course  of  our  studies  we  have  found  many  animals  that 
recognize  their  kind  and  more  or  less  definitely  associate  with 
them.  This  reaches  a  very  high  plane  in  the  bees  and  ants. 
Similar  social  life  is  to  be  found  in  all  the  primates,  but  it  is  not 
so  well  organized  among  the  lower  primates  as  among  the  ants. 

In  man,  even  in  primitive  man,  these  social  instincts  are 
well  shown.  They  are  complex,  just  as  in  the  bees.  They 
involve  the  mating  instinct,  the  care  of  young,  the  storing  of 
food,  the  finding  of  shelter  and  safety,  as  well  as  the  instincts  of 
general  gregariousness  found  in  many  forms.  These  associative 
instincts  lead  to  the  home,  to  marriage,  to  the  family,  and  later 
to  the  larger  family  or  group  made  up  of  the  immediate  kindred. 
Thus  tribes  and  clans  and  nations  come  to  be  bound  together. 
It  is  not  claimed  that  this  is  the  only  way  in  which  human  socie- 
ties may  be  built  up,  but  there  can  be  no  question  that  it  has 
been  so  built  up  in  many  instances. 

In  this  growing  complexity  of  society,  customs  and  regula- 
tions and  special  institutions  to  accomplish  certain  ends  spring 
up  just  as  the  division  of  labor  arose  in  the  bee  colony.  These 
demands  of  society  gradually  mould  the  individuals  and  their 
social  ideals.  Next  to  the  hand  and  to  language,  probably  the 
social  relations  of  man  have  been  important  in  training  his  mind 
and  developing  his  brain.  It  is  easily  seen  that  the  higher 
qualities  of  man,  as  sympathy,  love,  unselfishness,  heroism,  and 
self-sacrifice,  are  the  qualities  of  mind,  or  "heart"  as  we  some- 
times say,  that  would  be  given  prominence  in  the  home  and  the 
other  really  social  institutions.  The  study  of  man  and  his 
social  life  is  known  as  Sociology. 

472.  Education  and  its  Place  in  Human  Development— 

Education  is,  in  general,  the  development  of  the  individual  in 
such  a  way  that  he  will  be  able  to  adjust  and  to  readjust  himself 
rightly,  in  the  light  of  his  whole  nature,  to  the  essential  factors 
of  his  environment.  This  means  always  to  make  the  right  re- 
sponse to  the  stimuli.  This  adjustment  has  come  slowly  in  the 
race,  step  by  step  in  the  experience  of  each.  Individual  educa- 
tion in  mankind  is  an  effort  to  give  the  child  a  short  cut  to  the 


MAN  471 

best  that  has  been  discovered  by  the  race  in  its  history,  so  that 
it  will  not  be  necessary  for  him  to  get  it  all  by  experience.  Lan- 
guage is  again  the  vehicle  that  makes  this  possible.  Some  edu- 
cation is  possible  through  sight  and  imitation  of  parental  actions, 
and  there  is  probably  a  certain  amount  of  such  education  in 
many  of  the  lower  animals.  Confidence  in  the  parents,  imita- 
tion, and  curiosity  are  important  individual  instincts  underlying 
the  education  of  the  child.  The  long  dependence  of  the  child 
on  the  parents,  the  close  relations  of  the  home,  the  warmth  of 
sympathy  in  the  parental  feeling,  all  enter  in  furnishing  the 
motive  and  the  opportunity  for  the  training.  That  this  educa- 
tion is  efficient  is  shown  by  the  fact  that  the  individual  youth 
in  a  period  of  twenty  or  twenty-five  years  may  be  brought  to  a 
knowledge  of  the  most  important  experiences  of  the  human  race; 
of  the  great  implements  of  human  progress,  as  spoken  and  writ- 
ten language,  knowledge  of  nature's  laws,  and  the  relations  of 
numbers;  and  of  the  great,  partly  natural  and  largely  artificial 
structure  which  we  call  human  society,  as  well  as  of  the  modes  of 
behavior  necessary  to  meet  its  demands. 

These  have  required  thousands  of  years  to  build.  Most 
of  them  have  not  become  instinctive.  It  is  the  triumph  of 
human  attainment  that  so  much  can  be  imparted  in  so  short  a 
time. 

473.  Man's  Relation  to  Nature. — There  is  nothing  in  what 
we  can  learn  about  man  that  suggests  that  he  is  not  just  as 
dependent  on  the  natural  laws  of  his  own  being  and  of  the  en- 
vironment about  him  as  any  other  animal  in  the  animal  kingdom. 
He  starts  in  the  same  humble  way,  as  a  single  cell;  he  has  the 
same  powers  of  growth  and  development,  but  he  must  have  con- 
ditions favorable  to  them.  To-day  is  always  the  child  of  yester- 
day, just  as  with  the  other  animals.  He  has  the  like  diseases; 
he  is  affected  with  similar  parasites;  he  has  enemies  among 
the  animals  just  as  is  true  of  the  others.  Equally,  he  depends 
on  them  for  his  food.  The  same  struggle  for  existence  and  the 
survival  of  the  fittest  that  we  find  in  all  of  life  can  be  traced  in 
much  of  human  history. 

Man,  however,  has  made  a  mastery  of  nature  which  no  other 


472  ZOOLOGY 

forms  have  been  able  to  do.  Probably  through  his  wits  and 
his  supple  hands,  rather  than  by  strength,  he  held  his  own 
against  the  powerful  mammals  which  preceded  him  on  the 
earth.  By  the  same  means,  but  in  increasing  degree  he  holds 
that  mastery  to-day.  Through  his  wits,  again,  and  his  wonder- 
ful hands  he  has  managed  to  use  the  inorganic  forces  of  nature 
as  no  other  animal  has  or  can  do.  Through  his  wits,  and  most 
of  all  through  his  growing  sympathies  and  unselfishness,  he  bids 
fair  to  build  up  a  society,  based  on  friendship  and  love,  which 
will  substitute  cooperation  for  competitions  in  the  broader  rela- 
tions of  life  just  as  it  has  already  done  in  the  home  itself. 

474.  The  Artificial  Surroundings  of  Man. — In  what  we  call 
civilization  man  has  so  controlled  the  natural  conditions  as  to 
create  for  himself  an  environment  which  is  greatly  different 
from  that  under  which  man  first  lived.     Clothes,  houses,  cities, 
fire,  seasoned  foods,  stimulants  are  terms  which  suggest  some 
of  these  artificial  elements  that  have  entered  into  man's  life. 
Just  what  their  final  effect  will  be  on  the  body  and  mind  of  man 
no  one  can  tell.     Many  of  the  most  terrible  diseases  to  which 
man  is  subject  are  the  diseases  of  civilization.     That  his  wits 
will  continue  to  enable  him  to  meet  the  new  problems  which  he 
brings  on  himself,  as  he  has  met  the  natural  ones,  we  may  well 
believe.     But  unquestionably  we  must  realize  that  his  greatest 
task  is  to  use  his  increasing  mastery  of  nature  in  meeting  the  new 
difficulties  which  his  own  complex  and  artificial  civilization  is 
bringing  upon  him. 

475.  The  Age  of  Man  on  the  Earth. — There  is  no  sure 
knowledge  of  when  man,  in  his  present  form,  first  appeared 
on  the  earth.     It  is  known  that  man  lived  in  Europe  in  the 
later  portion  of  the  Ice  Age,  and  quite  probably  earlier.     His 
implements  of  stone  are  found  along  with  the  remains  of  such 
animals  as  the  cave-bear  and  mastodon  and  man-like  apes, 
either  extinct  now  or  found  only  in  tropical  regions.     The  oldest 
of  these  human  remains  are  found  in  southern  Europe  and  in 
northern  Africa.     In  a  limestone  cave  in  the  Neanderthal  in 
Germany  fossil  remains  have  been  found  of  a  man  believed  to  be 
much  more  primitive  than  any  other  known.     In  other  words 


MAN  473 

this  is  regarded  as  an  extinct  species;  one  of  several  now  extinct 
types  of  men  that  have  been  discovered.  We  have  signs  of  the 
gradual  improvement  of  his  stone  implements  from  rough  to 
smooth;  the  introduction  of  other  material  as  bone  and,  later, 
copper,  bronze,  and  iron.  It  is  only  after  the  thousands  of  years 
of  this  primitive,  unrecorded  history  that  we  come  to  the  his- 
tory of  such  well-separated  nations  as  those  of  the  Euphrates 
and  Nile  valleys,  whose  monuments  and  inscriptions  are  believed 
to  take  us  back  6,000  years  or  more. 

476.  The  Principal  Types  of  Men  and  Their  Distribution.— 

It  is  not  perfectly  certain  that  all  the  men  of  to-day  should  really 
be  placed  in  one  species,  although  if  we  follow  them  back  far 
enough  they  may  have  had  a  common  origin.  The  pigmies  of 
Africa  or  the  Negritos  of  the  Philippines  are  more  removed  from 
the  Caucasian  than  is  true  of  many  separate  species  of  the  wild 
animals,  and  the  differences  are  both  characteristic  and  constant. 
The  varieties  of  men  are  innumerable,  but  there  is  a  tendency 
to  group  them  all  under  three  main  heads  which  we  may  call: 
(i)  The  white  or  Caucasian;  (2)  the  yellow  or  Mongolian,  and 
(3)  the  black  or  African.  To  these  are  often  added  the  red  or 
native  American,  and  certain  island  and  peninsular  types  that 
do  not  agree  very  well  with  any  of  the  others.  The  most  striking 
external  differences  are :  The  color  of  the  skin,  the  structure  and 
appearance  of  the  hair,  the  form  of  the  nose  (which  is  also 
used  in  distinguishing  the  apes),  the  form  of  the  jaw  and  skull, 
and  the  character  of  the  language.  Under  the  Caucasian  race 
are  included  the  wavy-haired  peoples,  as  the  chief  European 
peoples,  the  Egyptians,  the  Jews,  the  Arabs,  the  East  Indians, 
and  the  peoples  of  the  Caucasus  who  give  the  name  to  the  race. 
The  Mongolians  would  include  the  straight-haired,  yellowish 
and  brownish  varieties,  as  the  Chinese,  Mongols,  Manchus, 
Tartars,  Japanese,  Turks,  Finns,  and  possibly  the  native 
American  tribes,  as  Esquimo,  Indians,  and  the  South  and 
Central  American  peoples.  The  black  or  negro  race  includes 
many  tropical  forms  dark  in  color  and  with  wavy  or  kinky  hair. 
Such  are,  the  dwarf  Negrillos  of  central  Africa;  the  Hottentots 
and  Bushmen;  the  Negroes  of  the  Nile,  of  the  Senegambia,  and 


474  ZOOLOGY 

of  Guinea;  the  Caffres,  Zulus,  and  other  tribes  of  the  east  coast; 
and  many  others. 

The  great  number  of  these  human  races,  or  varieties,  is  an 
evidence  of  the  long  time  man  has  been  on  the  earth  and  of  his 
adaptability.  They  have  arisen  and  have  been  increased  by  the 
natural  tendency  to  vary,  by  the  effects  of  climate,  by  migration 
and  isolation,  by  interbreeding,  by  conscious  selection  of  mates 
in  accordance  with  local  standards  of  attractiveness,  and  the  like. 
This  phase  of  zoology  is  known  as  Ethnography. 

477.  Topics  for  the  Library. — i.  Study  the  effects  of  tropical 
conditions  on  the  human  race,  as  judged  by  the  races  of  men  now 
found  there:  Effects  on  skin;  on  physical  and  mental  states;  on 
industrial  and  social  life. 

2.  Study  similarly    the   effects   of   life   in   the   temperate 
zones.     What  is  the  value  to  man  of  the  alternation  of  summer 
and  winter,  in  encouraging  thrift  and  foresight;  in  the  perma- 
nency and  character  of  homes,  etc.     The  effects  of  these  things 
on  stability  of  government  and  of  social  institutions  generally. 

3.  Similarly  study  the  conditions  of  extreme  cold,  as  seen 
in  the  life  of  the  Esquimos,  Finns,  Laplanders,  and  the  like. 

4.  Where  were  the  highest  examples  of  aboriginal  civilization 
on  the  American  continent  when  it  was  discovered  ?     Describe. 

5.  Describe  the  steps  whereby  the  clan,   the  tribe,   and 
the  nation  may  be  built  up  with  the  home  as  the  starting-point. 
What  new  personal  qualities  are  called  for  and  cultivated  by  the 
social  life  of  the  tribe  ? 

6.  Illustrate  the  effects  of  the  physical  geography  of  the 
earth  on  man's  life  and  history.     Examples:  Egypt;  Greece; 
Palestine ;  the  Mediterranean  Sea ;  the  depression  formed  by  the 
Hudson  River  and  Lake  Champlain;  the  Allegheny  mountains; 
effects  of  continents  on  races. 

7.  On  the  whole,  does  it  seem  that  man  has  been  sub- 
ject to  the  same  influence  by  external  conditions  which  we  have 
found  in  the  lower  animals?     Illustrate. 

8.  Human  migrations.     Cite  some  of  those  that  belong  to 
recorded  history  and  look  at  their  causes  and  results. 


MAN  475 

9.  What  is  the  zoological  character  of  mixed  or  hybrid  races 
of  men? 

10.  What  is  the  human  population  of  the  earth  estimated  to 
be?     How  is  it  believed  to  be  distributed  among  the  leading 
races?     Which  is  growing  with  greatest  rapidity?     What  rea- 
sons can  you  assign  ? 


CHAPTER  XXVI 

THE  DOCTRINE  OF  EVOLUTION  AND  RELATED  IDEAS 

478.  In  the  preceding  pages  the  words  evolution  and  devel- 
opment have  been  used  frequently,  but  no  effort  has  been  made 
to  define  or  justify  them.     In  all  that  has  been  said  it  has  been 
assumed  that  the  animals  now  on  the  earth  have  come  to  their 
present  condition  of  variety  and  complexity  by  natural  growth 
and  development,  rather  than  by  outright  creation  as  we  find 
them  now.     This  has  been  assumed,  not  because  there  can  be 
any  complete  demonstration  of  the  view,  but  because  it  is  more 
in  accordance  with  the  facts  as  we  find  them  than  any  other 
theory  which  has  been  offered. 

The  student  is  now  in  a  position  to  study  the  question  of 
evolution  more  broadly,  and  to  appreciate  a  more  full  statement 
of  it  and  some  of  the  discoveries  and  inferences  most  closely 
related  to  it. 

479.  The  Meaning  of  Evolution. — There  is  a  good  deal  of 
haziness  in  the  thought  of  people  generally  as  to  just  what  the 
zoologist  means  by  evolution.     Evolution  is  not  in  any  sense  a 
cause;  it  is  a  term  for  a  process,  for  the  way  in  which  present 
conditions  have  come  about.     Briefly,  the  most  important  ele- 
ments in  the  thought  of  evolution  are  gradualness  and  natural- 
ness.    In  more  detail,  the  following  features  may  be  said  to 
belong  to  the  idea  of  evolution : 

1 .  All  life,  so  far  as  we  can  know,  has  come  from  pre-existing 
life.     This  year's  animals  are  descended  from  those  of  last  year; 
they  from  those  of  the  year  before,  and  so  on  back. 

2.  All  animals  are  subject  to  change.     Offspring  are  never 
just  like  the  parents.     If  given  time  enough  animals  may  thus 
change  in  any  degree.     New  species  may  come  from  old  by 
change. 

3.  The  animals  of  the  present  time  are  descended  from 
simpler,  more  generalized  ones;  and  these  from  still  earlier  types. 

476 


DOCTRINE  OF  EVOLUTION  AND  RELATED  IDEAS        477 

So  even  the  most  complex  animals  of  the  present  have  arisen 
ultimately  from  ancestors  as  simple  as  the  simplest. 

4.  All    animals   have   fundamental    likenesses.     Some    are 
more  alike;  some  are  less  so.     The  fundamental  likenesses  mean 
kinship. 

5.  The  process  of  life  is  gradual  rather  than  sudden,  although 
the  rapidity  of  it  may  differ  at  different  times ;  it  is  natural  rather 
than  supernatural;  it  is  subject  to  the  same  laws  of  cause  and 
effect  which  operate  in  chemistry  and  physics,  and  is  not  lawless 
and  arbitrary. 

6.  On  the  whole,  the  life-processes  result  in  a  closer  and  more 
perfect  adjustment  of  organisms  to  one  another  and  to  the  inor- 
ganic environment. 

480.  Evidences  for  the  Development  Theory. — Biologists 
generally  are  agreed  as  to  the  fact  of  evolution,  and  there  is  no 
longer  any  direct  search  for  proofs  of  the  belief.     Any  disagree- 
ment among  them  is  in  respect  to  the  manner  in  which  evolution 
has  come  about,  and  the  present  search  is  for  the  causes  and  the 
factors  which  produce  it.     Many  people,  however,  look  with 
some  suspicion  on  the  theory.     For  this  reason  the  student 
should  have  before  his  mind  some  of  the  classes  of  facts  that 
have  convinced  biologists  of  its  truth. 

481.  Variability  as  an  Evidence. — The  changeableness  of 
organisms  is  the  fact  that  makes  it  impossible  for  the  biologist 
to  deny  evolution.     Every  day  we  see  differences  in  organisms 
of  the  same  species,  which  differences  have  been  brought  about 
by  differences  in  the  surroundings,  by  the  behavior  of  the  organ- 
isms .themselves,    by  cultivation    by   man,   or  by  something 
inherited  from  their  parents.     We  know  that  man  can  take  ad- 
vantage of  these  differences  and  can  select  certain  types;  can 
cultivate  and  select  again  in  such  a  way  as  to  get,  in  a  few  gen- 
erations of  cultivation  and  breeding,  animals  strikingly  different 
from  those  with  which  he  started.     After  a  certain  time  these 
new  forms  seem  to  breed  reasonably  true,  and  a  new  race  is  said 
to  be  established.     In  this  way  the  different  breeds  or  varieties 
of  dogs,  pigeons,  chickens,  and  many  other  domestic  animals 
have  apparently  arisen.     This  is  not  merely  a  proof  of  evolution; 


ZOOLOGY 

it  is  evolution.     Any  one  who  believes  in  this  is  an  evolutionist 
by  just  that  much. 

There  can  be  no  reasonable  doubt  that  just  this  kind  of 
thing  is  happening  in  nature,  without  the  help  of  man.  It 
cannot  take  place,  however,  so  rapidly  as  when  man  deliberately 
aids  the  process,  by  artificial  selecting  and  breeding  according 
to  his  preference,  and  then  eliminates  those  that  he  does  not  want. 
It  sometimes  happens,  both  in  nature  and  in  cultivation,  that 
large  variations  (''sports"  or  "mutations")  appear  suddenly, 
and  breed  true  in  succeeding  generations.  These  marked  varia- 
tions are  not  so  frequent  as  the  slighter  ones,  but  seem  to  be 
more  persistent  when  they  once  appear. 

482.  Evidences  from  Geographical  Distribution. — In  the 
wild  state,  the  changes  in  animals  are  too  slow  for  us  to  detect, 
within  a  few  human  generations,  that  there  has  been  any  change 
in  a  species;  yet  we  do  have  some  evidence  on  a  broader  scale 
which  is  of  a  nature  quite  similar  to  that  in  the  last  section. 
In  the  way  in  which  animals  occur  on  the  face  of  the  earth  a 
great  many  interesting  factors  enter,  and  it  sometimes  happens 
that  we  get  some  good  indication  of  the  long-time  effects  of 
variation.  Sometimes  we  find  two  forms  of  plant  or  animal 
flourishing  in  two  regions  that  are  separated  from,  each  other 
by  some  kind  of  a  barrier  which  does  not  allow  them  to  pass 
back  and  forth  and  thus  to  mingle.  These  forms  are  in  general 
similar,  and  yet  are  constantly  and  recognizedly  different. 
Sometimes  we  can  find  that  these  two  regions  (b  and  c)  have 
been  stocked  from  some  third  region  (a),  and  that  both  varieties 
are  apparently  descendents  of  the  same  ancestors.  Indexed,  we 
may  be  able  to  find  forms  of  all  grades  connecting  "a"  with  "b" 
on  one  side,  and  other  intermediate  types  connecting  "a"  with 
' '  c  "  on  the  other  side.  In  cases  like  this,  which  are  by  no  means 
infrequent,  it  is  believed  that  "b"  and  "c"  have  migrated  into 
their  respective  regions  and  in  becoming  adapted  to  their  new 
condition  of  life  have  so  evolved  as  to  become  different  from  their 
parent  stock  and  from  each  other. 

In  a  similar  way,  but  on  a  much  larger  scale  and  in  a  form 
too  complex  to  discuss  here,  we  find  evidences  of  change  and 


DOCTRINE  OF  EVOLUTION  AND  RELATED  IDEAS       479 

development  in  the  animals  of  the  great  continental  and  other 
natural  divisions  of  the  earth. 

483.  Evidences  from  Geological  History. — In  the  rock 
strata  of  the  crust  of  the  earth  we  find  abundant  plant  and  ani- 
mal remains  in  the  form  of  fossils.  In  the  most  recent  strata 
we  find  remains  similar  to  the  species  of  the  present  time,, 
whether  of  mollusks,  of  fishes,  or  of  mammals.  The  further 
back  we  go  in  the  earth's  history  the  less  similarity  we  find  be- 
tween the  fossils  and  the  present-day  life.  The  earlier  strata 
show  only  invertebrate  remains ;  later  the  fishes  appear,  although 
much  simpler  and  more  primitive  than  the  fishes  of  to-day.. 
Later  still  appear  amphibians,  reptiles,  mammals,  and  birds; 
and  last  of  all  man's  remains. 

The  conditions  are  just  what  we  should  expect  if  life  ap- 
peared on  the  earth  in  its  simpler  forms  and  gradually,  by 
evolution  through  the  ages,  became  complex  and  modern. 

These  things  are  not  only  true  in  a  general  way,  but  have 
been  found  to  be  true  of  the  special  types  of  animals.  For 
example,  the  fossil  remains  of  modern  horses  have  been  found 
in  recent  geological  strata.  The  modern  horse  has  only  one  toe 
on  each  foot  and  walks  on  the  end  of  that  toe.  He  has,  however, 
some  splints  on  either  side  of  this  digit  which  point  us  to  the 
history  of  his  toes.  In  the  geological  age  preceding  the  present 
we  find  the  remains  of  an  animal  clearly  like  the  horse,  in  which 
these  splints  are  larger  and  show  more  nearly  the  structure  of 
normal  toes.  By  tracing  the  conditions  backward  in  geological 
times,  links  have  been  found  which  connect  the  skeleton  of  the 
horse  of  the  present  day  with  an  animal  of  the  Eocene  period, 
which  had  four  toes  on  the  forefeet  and  three  toes  on  the  hind 
feet  and  was  little  larger  than  a  fox.  These  steps  are  so  com- 
plete that  expert  students  of  fossils  do  not  hesitate  to  regard 
that  we  have  a  fair  knowledge  of  the  ancestry  of  the  horse  for 
perhaps  millions  of  years.  Similar  series  of  gradual  changes 
are  shown  among  many  species  of  fossil  animals,  as  mollusks, 
Crustacea,  fishes,  birds,  and  mammals. 

All  this  is  entirely  without  explanation  if  animals  have  not 
been  subject  to  gradual  evolution  through  the  ages. 


480  ZOOLOGY 

484.  Evidences  from  General  Similarities  of  Structure.— 

In  studying  animals  we  need  to  keep  in  mind  not  merely  the 
varieties  and  dissimilarities  which  we  see  arising,  but  the  under- 
lying likenesses  as  well.  This  underlying  likeness  of  structure 
is  really  an  evidence  of  relationship;  which  is  another  way  of 
saying  that  animals,  in  spite  of  their  differences,  are  apparently 
descended  from  a  common  stock.  If,  for  example,  we  examine 
the  offspring  of  a  given  pair  of  parents,  as  a  litter  of  kittens  or  of 
puppies,  we  expect  these  individuals  to  be  more  alike  in  behavior, 
in  disposition,  and  structure  than  will  be  the  same  number  of 
offspring  of  totally  different  parents.  This  likeness  is  a  sign 
and  measure  of  their  kinship.  Two  spaniels  or  two  newfound- 
lands  are  more  like  each  other  than  either  is  like  to  any  other 
breed  of  dogs.  The  greater  similarity  is  again  the  sign  of  their 
greater  kinship.  But  the  spaniel  is  more  like  the  newfoundland 
than  either  is  like  the  wolf,  for  the  spaniel  and  the  newfoundland 
probably  belong  to  one  species  (Canis  jamilaris)  and  the  wolf 
to  another  (Canis  lupus). 

The  wolf  and  dog  are  more  alike,  and  hence  closer  akin, 
than  either  is  to  the  members  of  the  cat  family.  The  dogs  and 
wolf  belong  to  the  genus  Canis  and  the  cats  to  the  genus  Felis. 
This  is  the  way  we  express  their  degrees  of  difference.  But  in 
turn  the  dogs  and  cats  are  much  more  similar  to  each  other  than 
they  are  to  the  horse  and  cow.  Because  of  this  similarity  in 
structure  and  behavior,  the  dogs  and  cats  are  classed  together  as 
carnivores,  and  the  horse  and  cow  together  as  ungulates. 

With  all  their  differences  the  carnivores  and  the  ungulates 
have  many  fundamental  points  in  common.  They  are  all  warm- 
blooded, covered  with  hair,  have  mammary  glands,  carry  the 
young  in  the  uterus  attached  by  a  placenta.  Hence  they  belong 
to  the  subclass  placentals  and  to  the  class  of  mammals.  They 
have  similar  parts  to  their  skeletons,  similar  arrangements  of 
the  principal  muscles,  similar  structure  of  the  brain  and  central 
nervous  system.  Thus  it  might  be  shown  that  the  newfound- 
land  and  the  spaniel  are  similar  to  all  the  vertebrates,  and 
finally  to  all  the  animals. 

If  the  similarities  of  structure  in  a  litter  of  kittens  is  a  sign 
of  kinship,  we  may  equally  believe  that  the  similarities  between 


DOCTRINE  OF  EVOLUTION  AND  RELATED  IDEAS        481 

the  dog  and  the  cow  are  also  evidences  of  kinship ;  and  that  their 
differences  mark  a  gradual  evolution  from  a  common  stock  in 
special  directions  and  in  adjustment  to  special  modes  of  life 
through  long  ages  of  time. 

485.  Evidences  from  Rudimentary  Organs. — It  often  hap- 
pens that  some  animals  possess  organs  in  only  a  slight  or  rudi- 
mentary way,  which  in  other  animals  are  well  developed  and 
useful.     In  the  rabbit,  for  example,  there  is  a  pouch  called  the 
caecum  at  the  junction  of  the  small  and  the  large  intestine.     It 
is  several  inches  in  length,  well  supplied  with  glands,  and  proba- 
bly of  considerable  value  in  digestion.     In  many  mammals  this 
structure  is  much  reduced,  and  in  man  it  is  only  found  as  a 
"vermiform  appendix,"  which  certainly  has  no  such  value  as  it 
has  in  the  rabbit,  and  is  thought  by  many  physiologists  to  be  a 
positive  menace. 

Similarly,  in  many  mammals  there  are  certain  muscles  by 
which  the  external  ear  is  moved  and  directed  so  as  to  catch  the 
sound  waves.  In  man  these  muscles,  though  present,  are  so 
reduced  as  to  be  of  no  value.  Most  animals  have  many  rudi- 
mentary remnants  of  organs  which  are  useful  in  other  apparently 
related  animals.  It  is  said  that  man  alone  has  several  hundred 
such  rudimentary  structures.  The  rudimentary  eyes  of  fishes 
and  Crustacea  in  caves,  and  the  almost  or  entirely  reduced  organs 
of  many  parasites  are  mute  testimony  of  the  loss  of  organs  once 
useful,  through  changed  life  conditions.  In  other  words, 
vestigial  organs  are  also  evidence  of  evolution  of  animals  into 
adjustment  with  the  surroundings. 

486.  Evidences    from    Embryology. — The    study    of    em- 
bryology— the  course  of  life  in  the  individual — has  probably 
furnished  us  the  most  suggestive  evidence  of  the  evolution  of 
animals.     The  main  facts,  and  the  use  that  has  been  made  of 
them  in  reaching  our  conclusions,  may  be  expressed  in  a  few  brief 
statements.     The   student   must   go   to  more  extended  texts 
for  the  complete  handling  of  this  complex  but  most  interesting 
subject. 

Some  important  facts  of  individual  development  are : 
i.  Each    individual    animal    (with    certain    exceptions    in 
31 


482 


ZOOLOGY 


non-sexual  reproduction),  no  matter  how  high  in  the  scale 
of  life,  starts  its  life  as  a  single  cell,  similar  in  many  respects 
to  the  permanent  single-celled  protozoans,  except  that  it  has 
the  power  of  developing  rapidly  into  its  own  peculiar  species. 
(See  Fig.  250,  7.) 

FIG.  250. 
Mammalia, 


<5.  XfotochorA 


3.  Sey  mentation* 


.  Gtzstrulet 


l.O/ie-celted, 


FIG.  250.  A  diagram  to  suggest  the  way  in  which  individuals  of  the  different  phyla  have 
parallel  development  for  a  while,  and  later  diverge  in  their  own  particular  way.  For  example,  an 
Amphibian  begins  life  as  a  single  cell,  similar  to  the  simple  protozoa  (i);  later  it  forms  a  mass  of 
cells  similar  to  some  of  the  colonial  protozoa  (2) ;  in  forming  a  gastrula  (3)  it  is  going  through  a 
stage  similar  to  the  Hydra,  which  is  a  kind  of  permanent  gastrula.  Amphibians  run  parallel  to  the 
fishes  in  development,  in  the  formation  of  a  body  cavity  (4),  in  forming  segments  (5),  in  develop- 
ment of  notochord  (6),  in  the  presence  of  gills  in  early  life  (7);  but  part  company  with  the  fishes  in 
the  fact  that  they  lose  the  gills  and  develop  lungs  (8). 

Questions  on  the  figure. — Trace  out  other  instances  of  parallelism  until  the 
purpose  and  meaning  of  the  diagram  become  clear.  Then  find  other  facts  with 
respect  to  development  of  animals  and  show  how  the  diagram  cannot  do  justice 
to  the  whole  truth.  Compare  diagram  with  text,  sections  486  to  487. 

2.  All  higher  forms  agree  likewise  in  the  next  step  of  their 
development,  which  consists  in  the  division  of  this  single  cell 


DOCTRINE  OF  EVOLUTION  AND  RELATED  IDEAS        483 

into  a  simple  mass  of  cells.  This  is  known  as  the  morula,  which 
is  not  unlike  the  adult,  and  highest  stage,  of  such  types  as  Eudor- 
ina  and  Volvooc  (colonial  protozoa,  Fig.  250,  2). 

3.  Practically  all  the  higher  animals  pass  next  through 
a  stage  in  which  the  cells  become  differentiated  into  two  layers, 
more  or  less  well  defined  and  arranged  in  a  kind  of  double  sac 
— ectoderm  on  the  outside  and  entoderm  on  the  inside  (see  Fig. 
13 ,  A^) .  This  is  known  as  the  gastrula.  Animals  like  the  adult 
Hydra  (Fig.  81)  are  really  a  kind  of  permanent  gastrula — some- 
what modified  in  form  to  be  sure,  but  a  gastrula  nevertheless. 

The  facts  thus  far  stated  may  be  taken  as  suggesting  the 
following  conclusions: 

1.  All   organisms,   even  the  highest,   begin  life   at   essen- 
tially the  same  point;  that  is,  as  a  single  cell.     This  similarity  of 
individual  origin  indicates  their  fundamental  kinship  and  simi- 
larity of  racial  origin. 

2.  The  development  of  all  the  forms  above  the  protozoa 
is  parallel  for  at  least  a  brief  period;  that  is,  through  the  morula 
and  the  blastula  stages  (Fig.  13,  3  and  4).     This  parallelism  of 
development  added  to  the  similarity  of  origin  points  even  more 
strongly  to  their  kinship. 

3.  There  is  tendency  for  some  forms  to  drop  out  of  the  race 
and  to  become  permanently  fixed  about  certain  of  these  stages: 
as  most  protozoa  at  the  single-celled  stage,  Volvox  at  the  morula 
stage,  and  sponges  and  hydra  at  the  gastrula  stage.     Others  go 
on  and  introduce  new  steps  of  differentiation  before  reaching  their 
adult  development  (Fig.  250:  1-9). 

487.  Careful  study  of  the  later  stages  of  development  of 
the  higher  animals  gives  us  further  illustrations  of  these  truths 
and  enables  us  to  state  even  more  broadly  the  principles  deduced 
above.  For  example,  we  find  that  insects  have  a  parallel  course 
of  embryonic  development  in  which  the  great  body  of  insects 
agree  (Fig.  250).  The  same  is  true  of  the  vertebrates.  One  in- 
sect agrees  with  another  insect  in  the  mode  of  its  development 
more  nearly  up  to  the  complete  adult  character  than  an  insect 
will  agree  with  an  echinoderm,  or  either  with  a  vertebrate. 
In  a  similar  way,  all  vertebrates  have  a  course  of  develop- 


484  ZOOLOGY 

ment  parallel  for  a  longer  time  than  they  parallel  any  other 
group  (Fig.  250,  1-7). 

Furthermore,  the  same  principle  applies  to  the  subgroups 
of  the  vertebrates,  or  of  any  other  branch.  The  vertebrate 
development  begins  to  differ  from  the  insect  development  lower 
down  than  the  divergence  of  the  reptiles  from  the  birds,  or  the 
mammals  from  the  fishes  (Fig.  2  50,  8) .  The  parallelism  between 
birds  and  reptiles  ends  earlier  than  that  between  one  type  of 
birds  and  another,  or  between  man  and  the  other  mammals. 
Finally  within  a  species,  as  of  grasshoppers,  of  reptiles,  or  of 
rabbits,  the  course  of  development  of  different  individuals  con- 
tinues identical  through  to  the  mature  form.  It  is  just  those 
forms  that  seem  most  similar  in  structure  (section  484)  in  which 
the  embryonic  development  is  most  nearly  identical.  The  two 
forms  of  similarity,  structure  and  embryology,  strengthen  the 
idea  of  real  kinship.  Hence  we  have  reached  the  conclusion  that 
all  parallelisms  in  the  fundamentals  of  individual  history  mark 
some  degree  of  relationship.  The  longer  and  more  profound  the 
parallelism  the  closer  the  relationship.  A  careful  study  of  the 
diagram  (Fig.  250)  will  serve  to  make  this  point  more  clear. 

488.  The  Biogenetic  Law. — Out  of  such  considerations  as 
these  has  come  one  of  the  most  important  and  far-reaching  laws 
that  the  biologists  have  ever  stated.  It  is  as  follows:  "Each 
individual  animal,  in  passing  from  the  egg  to  the  adult,  repeats 
in  an  abbreviated  way,  in  a  few  days  or  years,  many  of  the  steps 
taken  by  the  race  to  which  it  belongs,  in  its  evolution  from  its 
single-celled  ancestors  to  its  present  condition."  Put  briefly,  it 
reads :  Individual  history  is  a  brief  recapitulation  of  race  history. 

Why  can  the  individual  egg  cell  of  a  frog,  in  a  few  months, 
pass  through  a  morula,  a  blastula,  a  water-breathing  legless  fish- 
like  stage,  and  into  an  air-breathing,  four-legged  adult?  Be- 
cause its  ancestors  were  first  single-celled  animals,  and  through 
millions  of  years  and  countless  generations  gradually  developed 
first  into  a  mass  of  cells,  later  into  a  gastrula-like  stage,  and  later 
still  for  long  ages  breathed  by  gills,  like  the  fishes,  and  last 
of  all  became  what  they  now  are — water-breathing  tadpoles 
with  the  power  to  breathe  air  in  the  adult  life. 


DOCTRINE   OF   EVOLUTION   AND   RELATED   IDEAS 


485 


FIG.  251. 


UNGULATES 


UNGUICULATES 


MODERN  BIRDS 

CURSORES 


TOOTHED  BIRDS 


PLACENTA LS 

MARSUPIALS 
\ 

MONOTREMES 
\ 

MODERN  REPTILES 


\ 


ARCHAEOPTERYX 


CYCLOSTOMES 


ELASMOBRANCHS 

CEPHALOPOOS 

INSECTS 


ARACHNIDA 


PRIMITIVE  CHORDATES 

IXH 


ECHINODERMS 

N. 

COELENTERATA 


GASTEROPODS 


LAMELLIBRANCHS 


PORIFERA 


UNSEGMENTED  WORMS 


DIPLOBLASTIC  METAZOA 


COLONIAL  PROTOZOA 

! 

SIMPLE    PROTOZOA 

PIG.  251.  A  diagram  designed  to  suggest  something  of  the  possible  relationships  of  the  various 
groups  of  animals.  The  vertical  position  indicates  roughly  the  relative  specialization  or  progress 
of'the  groups. 

Questions  on  the  figure. — Which  of  the  groups  seem  to  be  centers  from  which 
several  types  may  have  sprung?  What  does  this  mean?  Which  phyla  do  not 
seem  to  have  others  springing  from  them?  What  does  this  mean?  What  three 
theories^as  to  the  origin  of  the  Chorda ta  are  suggested  in  the  figure? 


486  ZOOLOGY 

Because  the  history  is  so  much  shortened,  only  some  of  the 
more  profound  stages  are  recapitulated,  and  the  advances  of  the 
race  in  thousands  of  years  are  compressed  into  moments  in  the 
individual  history.  Furthermore,  each  species  has  introduced 
idiosyncrasies  that  belong  in  no  way  to  the  race.  For  these 
reasons  the  general  truth  of  recapitulation  must  not  be  applied 
too  literally  or  widely. 

489.  The  Principal  Factors  Entering  into  Evolution. — As  was 

indicated  earlier,  biologists  no  longer  question  the  idea  of  the 
evolution  of  the  animal  kingdom.  They  are  now  seeking  to 
find  the  principal  elements  that  are  bringing  about  this  result 
and  how  each  contributes  to  the  process.  We  doubtless  have  not 
found  all  the  factors.  However,  those  mentioned  below  cer- 
tainly enter  into  the  explanation. 

Evolution  could  not  take  place  without  variation,  nor  with- 
out some  device  to  preserve  (i.e.,  repeat)  and  to  accumulate  these 
variations  from  generation  to  generation  after  they  arise.  Fur- 
thermore, evolution  seems  to  follow  rather  definite  courses. 
These  courses  are  not  haphazard.  They  have  led  toward  adap- 
tation of  the  organisms  to  their  most  pressing  surroundings. 
In  other  words  there  seems  to  have  been  selection  and  elimination 
of  variations,  and  thus  a  guidance  of  evolution. 

The  following  outline  may  aid  the  student  in  grasping  some 
of  the  important  elements  of  the  case. 

i .  Variation  makes  evolution  possible,  but  only  this'.  We  have 
learned  (§60)  that  body  cells  and  germ  cells  lead  a  parallel 
life  in  every  organism.  They  are  very  different  in  their  proc- 
esses and  their  fate.  Theoretically  variations  may  arise  in 
either  of  two  ways: 

A.  The  body  of  a  given  organism  may  change  by  the  direct 
effect  of  the  environment  upon  the  body  or  through  the  use 
or  disuse  of  its  organs. 

B.  Changes  may  occur  in  germ  cells. 

a.  Germ  cells  may  be  modified  by  their  environment, — 
which  is  chiefly  the  body  in  which  they  are  being  housed. 

b.  Germ  cells  may  be  modified  by  the  mingling  of  the  two 
different  germ  cells, — ovum  and  sperm, — that  united  to 


DOCTRINE  OF  EVOLUTION  AND  RELATED  IDEAS        487 

form  them.     This  new  compound  is  different  from  any 
ever  formed  before. 

2.  Preservation,  repetition,  and  accumulation  of  variations  from 
generation  to  generation.     Only  this  can  insure  evolution.     There 
are  two  problems: 

a.  Preservation  of  fluctuations  that  come  to  the  body.     If 
there  is  an  early  separation  of  germ  cells  and  body  cells  and  they 
lead  a  parallel  course  of  development,  it  becomes  a  problem 
whether  a  special  quality  that  is  gained  by  the  body  cells  in 
their  life  can  be  imparted  to  their  cousins  among  the  germ  cells. 
Unless  they  can  do  this  it  is  clear  that  these  changes  cannot  be 
transmitted  to  the  next  generation,  since  the  body  cells  do  not 
themselves  pass  over.     A  body  may  have  acquired  a  very  strik- 
ing quality  without  any  part  of  this  quality  being  found  in  the 
germ  cells. 

b.  Preservation  of  changes  that  come  to  the  substance  of  the 
germ  cells.     Clearly,  since  the  germ  cells  make  the  next  genera- 
tion, changes  in  these  cells  may  very  well  influence  both  the  germ 
and  body  cells  of  the  next  generation.     Biologists  are  coming 
more  and  more  to  feel  that  this  is  the  really  great  field  of  heredi- 
tary  influence,    and  that  evolutionary   studies  must  concern 
themselves  increasingly  with  the  history  of  these  cells  and  the 
kinds  of  influences  that  can  change  them. 

3 .  The  Guidance  of  Evolution . — This  might  take  place  in  either, 
or  both,  of  two  ways. 

a.  Variation  might  itself  be  guided  from  within,  so  that 
chiefly  suitable  or  appropriate  variations  shall  occur. 

b.  Variations  may  have  any  range,  and  the  environment 
through  its  life  and  death  pressure  on  the  organisms  may  elimi- 
nate some  and  select  others  on  the  basis  of  fitness.     The  result 
would  be  to  make  evolution  follow  the  demand  of  the  environ- 
ment independently  of  the  range  of  the  variations. 

490.  Variability  and  Variation. — This  furnishes  the  materials 
of  evolution.  All  organisms  vary.  Variation  suggests  two  or 
three  things:  (i)  An  animal  may  differ  from  day  to  day  as  the 
result  either  of  its  own  activities  or  of  the  action  of  the  environ- 
ment upon  it;  (2)  it  may  differ,  at  any  stage  or  condition  of  its 


488  ZOOLOGY 

life,  from  what  its  parents  were  at  the  corresponding  stage;  and 
(3)  it  may  differ  also  from  its  brothers  and  sisters  even  at  the 
same  stage  of  life  and  under  similar  conditions.  Some  of  these 
changes  are  evidently  caused  by  conditions  outside  the  organ- 
ism; others  by  internal  conditions.  These  changes  make  evolu- 
tion possible.  We  do  not  believe  that  all  these  classes  of  changes 
enter  equally  into  evolution.  One  of  the  most  important  ques- 
tions of  the  modern  biologist  is  this:  "What  produces  the  varia- 
tion actually  found  in  individuals  of  a  species,  and  to  what  extent 
are  these  variations  due  to  internal  or  to  external  causes?" 

When  we  actually  study  variations  in  nature  we  find  them 
of  two  principal  kinds.  First,  and  chiefly,  we  find  what  are 
called  fluctuations  or  continuous  variations.  They  are  so  called 
because  they  tend  to  cluster  about  a  mean,  with  no  sharp  break 
in  the  series  from  the  lowest  to  the  highest.  We  should  find  this 
if  we  were  to  take  at  random  a  thousand  men  and  arrange  them 
on  the  basis  of  weight  or  height  or  intelligence.  A  curve  drawn 
to  show  their  distribution  would  show  the  greatest  number  near 
the  average  and  the  extremes  would  be  gradually  less  numerous. 
The  curve,  however,  would  be  a  gradual  one,  and  we  would  find 
cases  illustrating  all  degrees  within  the  normal  range. 

In  the  second  case,  however,  while  most  members  of  a 
species  will  arrange  themselves  as  above,  we  may  find  an  occa- 
sional individual  so  different  from  all  others  that  there  are  no 
' '  connecting  links. ' '  These  are  known  as  ' '  sports  "or  "  mutants ' ' 
and  illustrate  discontinuous  variation.  Experiment  shows  that 
the  mutations  are  much  less  frequent,  but  much  more  liable  to 
breed  true,  than  the  fluctuations.  In  the  case  of  fluctuations 
even  the  infrequent,  extreme  forms  produce  offspring  whose 
average  tends  to  be  not  about  the  extreme  parent,  but  rather 
about  the  mean  of  the  group.  The  offspring  of  the  mutants,  on 
the  other  hand,  have  their  mean  about  the  parent,  and  are  quite 
independent  of  general  mean. 

491.  Heredity, — the  Organic  Continuity  between  Genera- 
tions.— We  have  already  seen  (§131)  that  the  germ  cells,  ovum 
and  sperm,  are  the  organic  bridge  which  connects  the  body  of  one 
generation  with  that  of  the  next.  Just  as  new  qualities  cannot 


DOCTRINE  OF  EVOLUTION  AND  RELATED  IDEAS        489 

influence  evolution  until  they  are  represented  in  the  protoplasm 
of  the  germ  cells  (germ  plasm) ,  so  all  the  old  qualities  of  the  race, 
both  deep  and  superficial,  in  order  to  continue,  must  be  repre- 
sented in  this  germ  plasm.  When  the  germ  cells  have  united, 
inheritance  has  been  determined.  After  this  the  parent  may 
influence  the  development  of  the  new  individuals, — as  the 
mother  mammal  doubtless  does  while  the  young  is  carried  in  the 
uterus  or  nourished  by  milk;  but  neither  of  these  is  a  matter  of 
heredity  in  any  sense. 

In  those  organisms  in  which  there  is  no  union,  where  a  single 
germ  cell  may  develop  into  the  adult,  only  one  line  of  germ  plasm 
is  involved  and  heredity  seems  a  somewhat  simpler  thing.  How- 
ever, when  two  different  strains  of  germ  cells  unite  to  start  the 
new  generation,  every  cell  of  the  new  individual  receives  chromo- 
somes, and  probably  other  material,  that  came  by  way  of  both 
ovum  and  sperm.  It  is  believed  that  the  hereditary  qualities 
are  carried  in  some  way  by  these  chromosomes.  Chromosomes 
from  the  sperm,  carrying  definite  characteristics  of  the  father's 
line,  are  closely  mixed  in  the  same  nucleus  with  chromosomes 
of  the  egg  carrying  characteristics  of  the  mother's  line.  In 
what  way  will  these  diverse  complex  chemical  bodies  influence 
one  another  in  the  development  of  the  body  ?  Will  they  blend 
or  will  one  swamp  or  exclude  the  other?  If  they  blend,  will 
they  add  their  tendencies;  will  they  neutralize  each  other;  or 
will  their  union  give  rise  to  compounds  so  new  as  to  carry 
qualities  that  neither  parent  had ;  or  may  all  of  these  happen  with 
the  different  qualities?  These  are  the  questions  of  heredity. 
Only  recently  have  we  begun  to  get  real  statistical  and  breeding 
data  that  will  help  us  to  answer  them. 

492.  Mendel's  Experiments. — Gregor  Mendel,  an  Austrian 
monk,  published  in  1866  the  results  of  breeding  experiments 
conducted  by  him,  together  with  his  interpretation  of  these. 
His  work  was  unknown  until  1900.  Since  that  time  his  facts 
have  been  verified  and  enlarged,  and  his  conclusions  tested  and 
elaborated  until  we  now  include  under  "  Mendelism "  a  large 
body  of  exceedingly  important  facts  and  suggestive  theories 
and  principles. 


49°  ZOOLOGY 

Mendel  hybridized  varieties  of  garden  peas.  One  true- 
breeding  variety  is  dwarf  and  another  tall.  When  these  are 
artificially  crossed,  no  matter  which  variety  furnishes  the  pollen, 
all  the  offspring  are  like  the  tall  parent  in  height.  The  dwarf 
quality,  which  evidently  must  be  represented  in  the  offspring, 
does  not  show  in  the  body  at  all.  He  therefore  concluded  that 
tallness  was  dominant  over  dwarfness.  Dwarfness  is  said  to  be 
recessive. 

FIG.  252. 
TALL  (pure)  X  DWARF 


TALL.. 

(Hybrid) 


self-bred 

^  s 

TALL  (3 )  DWARF  ( i ) . . .  .  F2 


/  i  I 

TALL  (i)  TALL  (2)  DWARF F3 

(pure)  (hybrid)  (all) 

FlG.  252.  Diagram  illustrating  Mendel's  results  in  breeding  tall  and  dwarf  peas.  Pi,  Fz,  F», 
firtt,  second,  and  third  filial  generations.  The  first  generation  are  all  tall;  but  we  know  they  are 
impure  (hybrid)  both  because  we  know  the  parents,  and  by  their  offspring  when  bred  among  them- 
selves. The  dwarfs  (recessive)  when  they  once  appear  are  necessarily  pure,  and  breed  true.  The 
tails  of  Fj  generation  are  of  two  kinds  as  shown  by  their  offspring, — i  pure  to  2  impure.  The  figures 
in  parenthesis  indicate  relative  numbers.  All  generations  after  F,  are  self-bred. 

When  these  tall  hybrids  were  self -fertilized,  three-fourths  of 
their  offspring  were  tall  like  the  parents,  and  one-fourth  were 
dwarf  like  the  dwarf  grandparent.  In  the  case  of  the  dwarf 
offspring  the  parents  clearly  transmit  something  which  the 
parent  body  does  not  possess.  There  were  no  intermediate 
forms.  Further  breeding  showed  that  these  dwarfs  were  pure 
dwarfs  with  no  taint  of  tallness  on  them ;  for  when  self -fertilized 


DOCTRINE  OF  EVOLUTION  AND  RELATED  IDEAS 


491 


they  bred  true  dwarf  indefinitely.  Close-breeding  the  tails 
showed  them  to  be  of  two  kinds:  (i)  one-  third  of  them  (that  is, 
one-fourth  of  the  whole  offspring)  were  just  as  pure  as  the  dwarfs 
were,  and  continued  to  produce  tall  forms  indefinitely.  Two- 
thirds  of  the  tails  (one-half  the  total  offspring)  when  inbred 
had  three  tall  offspring  to  one  dwarf,  —  just  as  had  been  true  of  the 
first  hybrid  generation.  Further  inbreeding  of  the  hybrid  off- 
spring continues  to  repeat  these  proportions  indefinitely:-!  12:1, 
—that  is  one  pure  tall,  two  hybrid  tails,  and  one  pure  dwarf. 

FIG.  253. 

C  X  c     Parental  Generation 


C(c)     ist  Filial  Generation 
(Self-crossed) 


"C                      C(c)  ' 
\ 

C(c) 

c 

:        c 

.  \ 

c      c 

c 

c  

c.  . 

\          \ 
'C         CC    C(c)    C(c) 

cc           C 

1              \ 
cc           C 

C   C( 

c)    C(c)      a 

11             " 
Hybrids 
4 

r      cc 

Hybrids 
. 
C                           c 

FIG.  253.  A  general  formula  for  Mendelian  crosses.  C  represents  any  dominant  quality; 
c,  the  absence  of  the  quality,  or  the  recessive.  The  first  filial  generation  are  all  hybrids  that  look 
like  the  dominant  parent.  If  these  are  self-bred,  the  offspring  of  the  second  filial  generation,  F«, 
will  be  one-fourth  pure  dominant  (C),  one-fourth  pure  recessive  (c},  and  one-half  hybrids  which  look 
and  breed  like  their  hybrid  parents.  CC  and  C(c)  look  alike,  but  breed  differently.  Compare 
with  Fig.  252. 

Mendel  also  found  that  yellowness  of  seed  coat  was  dominant 
over  greenness;  that  smoothness  of  seed  was  dominant  and 
wrinkledness  was  recessive.  He  also  found  that  these  different 
qualities  were  inherited  absolutely  separately;  that  is  to  say 
that  the  yellow  or  green  and  the  smooth  or  wrinkled  quality 
of  the  seeds  might  be  transmitted  equally  in  connection  with 
either  the  dwarf  or  the  tall. 

The  results  stated  in  general  terms  might  be  expressed  as 
follows :  When  pure  bred  parents  differ  by  the  presence  or  ab- 
sence of  a  certain  heritable  quality,  all  the  offspring  of  the  first 
generation  will  be  like  one  of  the  parents  in  bodily  appearance. 


4Q2 


ZOOLOGY 


This  parent  and  its  quality  are  said  to  be  dominant.  When  the 
individuals  of  this  first  hybrid  generation  are  self-fertilized,  or 
crossed  with  each  other,  25  per  cent,  will  be  pure  and  like  the 
recessive  grandparent;  25  per  cent,  will  be  pure  and  like  the 
dominant  grandparent;  and  50  per  cent,  will  be  hybrids  like  the 
immediate  parents,  and  having  the  bodily  appearance  of  the 
dominant  grandparent. 

The  diagram  (Fig.  253)  will  aid  the  student  in  following  the 
general  results. 

493 .  Mendel's  Laws. — Three  laws  or  principles  were  deduced 
by  Mendel  from  his  experiments,  and  have  been  perfected  by 
later  workers. 

PIG.  254. 
Possible  Kinds  of  Male  Gametes 


T 


D 


TT 
Pure  tall 


T(D) 

Hybrid  tall 


T(D) 


DD 


1  Hybrid  tall  Pure  dwarf 


FlG.  254.  Diagram  showing  the  possible  kinds  of  crosses  of  the  hybrids  of  the  first  filial  genera- 
tion in  Mendel's  peas.  Each  hybrid  parent  may  produce  eggs  and  sperm  carrying,  when  they 
segregate,  either  tallness  or  dwarfness,  but  not  both.  These  in  the  long  run  will  be  equal  in  num- 
bers. Each  kind  of  sperm  will  have  equal  chances  of  uniting  with  either  kind  of  egg. 

Questions  on  the  figure. — Why  are  these  the  only  possible  gametes?  Follow 
out  the  details  of  the  diagram  and  see  just  why  the  lettering  in  the  squares  is  as  it 
is,  and  determine  the  quality  of  the  resulting  offspring  in  each  case.  Is  it  clear 
why  the  resulting  proportions  are  as  they  are? 

a.  The  principle  of  unit  characters.  This  suggests  that 
organisms  do  not  inherit  the  whole  parental  nature  as  a  unit, 
but  that  each  parental  quality  acts  as  a  unit  in  inheritance. 
Each  individual,  from  the  point  of  view  of  inheritance,  is  made 
up  of  many  independent  unit  characters  which  may  be  inherited 
in  any  combination. 


DOCTRINE  OF  EVOLUTION  AND  RELATED  IDEAS       493 

Later  researches  support  this  principle  in  the  main,  but  show 
that  these  unit  characters  do  influence  one  another.  Some- 
times they  are  linked  or  coupled  in  such  a  way  that  one  cannot 
be  inherited  without  another;  sometimes  they  repel  one  another 
so  that  they  cannot  be  inherited  together;  and  sometimes  a 
quality  that  appears  to  us  as  one  in  the  body  is  known  to  be 
made  up  of  two  or  more  unit  characters  on  the  germ  cells. 

.Fie.   255. 

TALL  PARENT  DWARF  PARENT 


THESE  PARENTS  FORM  PURE  GAMETES    f~\ 
EGGS  OR  SPERM  SPERM  OR\GGS 

£-—P 


GAM  \    ETES  SEGRE  /GATE 


PURE  TALL  HYBRID  TALL  PURE  DWARF 

T  (D)  DO 

FIG.  255.  A  diagram  to  show  graphically  the  formation  of  gametes,  dominance,  possible 
matinga,  segregation  of  qualities  in  the  gametes,  and  the  proportion  of  resulting  offspring,  in  the 
Pi  and  Pj  generations  of  the  tall  and  dwarf  peas.  Black  indicates  tallness;  white,  dwarfnesi. 
Black  outside  the  white  indicates  its  dominance.  TT,  a  plant  in  which  tallness  comes  from  each 
parent;  T(D),  in  which  tallness  comes  from  one  parent  and  dwarf  ness  from  the  other;  DD,  dwarf- 
ness  from  each  parent. 

Questions  on  the  figure. — Why  are  the  gametes  of  each  original  parent  figured 
alike?  Why  are  those  of  the  FI  parents  unlike?  Why  are  the  gametes  always 
either  white  or  black,  not  mixed  as  the  parents  may  be?  Carry  for  yourself  the 
figure  one  step  further,  that  is  to  the  P3  generation.  What  kind  and  proportion  of 
offspring  would  you  get  by  crossing  the  possible  gametes  of  TT  X  TT?  TT  X 
T(D)?  T(D)  X  T(D)?  T(D)  X  DD?  DD  X  DD?  Make  a  diagram  of  each. 

b.  The  principle  of  dominance.  There  are  determiners  in  the 
germ  plasm  which  stand  for  these  unit  characters  in  the  body. 
When  these  are  mingled  by  the  union  of  two  pure  germ  cells, 
one  of  these  (e.g.,  that  for  tallness  in  the  peas)  will  dominate  the 
other  so  that  the  latter  though  present  in  all  the  cells,  both  body 


494  ZOOLOGY 

cells  and  primordial  germ  cells  (see  §§49,  60),  has  no  part  in 
developing  the  body.  The  recessive  determiner  cannot  develop 
in  the  presence  of  the  dominant.  Whenever  the  dominant  deter- 
miner is  absent,  the  alternative  recessive  qualities  will  appear. 
This  explains  why  the  inbred  recessives  always  breed  true. 
They  would  not  show  at  all  if  there  were  any  taint  of  the 
dominant  quality. 

c.  The  principle  of  segregation,  or  purity  of  the  gametes. 
This  principle  asserts  that,  while  both  dominant  and  recessive 
determiners  are  to  be  found  in  every  primordial  germ  cell,  both 
determiners  cannot  go  into  one  sperm  or  one  ovum.  In  the 
process  of  maturing  ova  and  sperm  by  nuclear  division  the  two 
kinds  of  determiners  separate,  and  thus  an  equal  number  of 
sperm  and  ova  carry  the  dominant  and  the  recessive  characters. 
In  other  words,  while  the  body  of  an  organism  may  be  a  hybrid, 
the  eggs  and  sperm  are  always  pure. 

In  the  peas,  for  example,  if  T  represents  the  determiner  for 
tallness,  and  t  that  for  dwarf  ness,  one-half  the  egg  cells  of  the 
first  hybrid  generation  will  carry  T  and  one-half  will  carry  the 
recessive  t.  The  same  will  be  true  of  the  male  nuclei  of  the  pollen 
tube.  In  mating  the  chances  are  even  that  each  kind  of  male 
cell  will  fertilize  each  kind  of  female.  The  "checker-board" 
diagram  (Fig.  254)  will  show  the  probable  number  of  offspring  of 
each  possible  type. 

494.  The  Relation  of  MendePs  Conclusions  to  the  Process  of 
Maturation  of  Ova  and  Sperm. — The  student  will  realize  that 
these  views  of  Mendel  fit  in  remarkably  with  the  facts  that  were 
brought  out  (§49  and  §50)  relative  to  the  behavior  of  chromo- 
somes in  nuclear  division,  and  especially  in  the  divisions  in  the 
forming  of  sperm  and  ova.  It  is  not  possible  to  develop  the  sub- 
ject at  length  here.  These  two  lines  of  study, — experiments  in 
breeding,  and  studies  of  the  structure  and  behavior  of  the  minute 
parts  of  the  germ  cells, — supplement  one  another;  and  both  are 
throwing  increasing  light  upon  the  problems  of  inheritance.  It 
only  adds  to  the  appreciation  of  Mendel's  results  to  recall  that 
nothing  was  known  in  his  time  of  these  remarkable  facts  of 
cell  behavior. 


DOCTRINE  OF  EVOLUTION  AND  RELATED  IDEAS        495 

495.  Isolation. — When  new  variations  occur  it  is  clear  that 
they  will  have  a  better  chance  to  accumulate  and  result  in  a 
new  and  permanent  stock  if  for  any  reason  these  new  characters 
cannot  be  "swamped"  or  buried  by  crossing  with  the  more 
numerous  and  conservative  members  of  the  parent  stock. 

The  effects  of  geographic  or  physical  isolation  is  seen  on  is- 
lands which  have  been  populated  from  the  mainlands,  or  in 
neighboring  lakes,  or  in  valleys  between  which  there  are  barrier 
mountains.  In  such  conditions  the  animals  and  plants  are  dis- 
tinctly different  from  the  species  from  which  they  are  believed 
to  have  sprung,  and  those  on  different  islands  are  different  from 
one  another.  It  is  believed  by  many  observers  that  a  larger 
number  of  related  species  are  found  under  such  conditions  than 
where  there  is  free  interbreeding  and  migration. 

It  is  possible  also  that  there  are  internal  hindrances  to  mating 
which  would  operate  in  much  the  same  way  as  geographic  iso- 
lation. There  may  arise  some  instinctive  feeling  which  would 
prevent  mating  between  the  average  members  of  a  species  and 
individuals  that  had  varied  considerably  from  that  average. 
We  see  this  in  some  degree  in  human  races.  Or  changes  in  the 
copulating  organs  or  in  the  reactions  of  the  germ  cells  might 
render  such  free  intercrossing  impossible. 

Finally,  we  have  seen  in  Mendel's  experiments  that  certain 
unit  characters,  even  when  the  sperm  and  ova  unite  freely,  do 
not  permanently  coalesce  and  blend;  but  sooner  or  later  may 
separate  out  pure  again.  In  a  sense  this  is  a  case  of  physiological 
isolation  of  the  structures  that  carry  the  varying  characters 
even  in  the  union  of  germ  cells. 

We  may  say  then  that  the  varying  animals  may  be  kept 
apart  by  geographic  barriers;  or  the  germ  cells  may  be  kept 
apart  even  when  the  animals  live  freely  together;  or  the  char- 
acter-bearing determiners  may  be  kept  apart  even  when  the 
germ  cells  come  together. 

496.  The   Environment. — Assuming   that   variations  occur 
and  make  evolution  possible,  and  that  heredity  enables  some  of 
these  variations  to  pass  from  one  generation  to  another  and  thus 
to  accumulate,  and  that  isolation  keeps  the  various  individuals 


496  ZOOLOGY 

from  evening  up  and  thus  destroying  their  variations  by  inter- 
crossing, it  still  remains  to  find  the  factors  that  have  determined 
and  guided  the  actual  course  of  evolution  in  any  given  instance. 
Why  has  development  taken  the  course  it  has  ? 

Theoretically,  as  we  have  seen,  there  might  be  an  inherent 
tendency  in  living  matter  at  the  beginning  to  vary  or  evolve  in 
a  certain  direction.  Biologists  are  not  agreed  upon  the  exist- 
ence of  such  a  tendency.  On  the  other  hand,  the  environ- 
ment— meaning  the  total  external  conditions  of  the  life  of 
organisms — may  guide  or  direct  the  course  of  development. 
Any  guidance  must  come  from  the  one  or  the  other  of  these 
sources, — internal  or  external. 

The  environment  may  act  in  either  of  two  ways  to  mold 
evolution.  We  have  seen  in  the  first  place  that  the  environment 
does  act  directly  to  produce  specific  changes  in  organisms. 
If  these  changes  can  be  inherited,  this  will  be  a  most  important 
means  of  guiding  evolution.  It  is  not  certain,  however,  that 
these  direct  effects  of  the  environment  on  the  body  of  an  organ- 
ism can  be  transmitted  to  the  germ  cells.  External  conditions 
may  likewise  act  directly  upon  the  germ  plasm  so  as  to  produce 
changes  in  its  nature.  Doubtless  such  changes  would  perma- 
nently affect  future  generations. 

Whether  this  is  true  or  not,  there  is  another  effect  of  the 
environment  which  has  certainly  had  a  great  influence  on  the 
course  of  evolution.  The  principle  was  discovered  by  Darwin 
and  by  him  called  ' '  natural  selection. ' '  It  refers  to  the  fact  that 
the  struggle  for  existence  among  organisms  is  so  severe  that 
•some  will  inevitably  be  eliminated.  We  have  seen  that  this  is 
true  among  all  species.  A  hundred  are  born  of  two  parents: 
only  two  of  these,  on  an  average,  will  be  preserved.  Which 
will  be  successful?  In  the  long  run,  those  will  be  preserved 
which  are  best  adjusted  to  the  conditions  of  the  life  which  they 
meet.  This  fact  is  independent  of  how  they  came  to  be  ad- 
justed. This  is  known  as  the  "survival  of  the  fittest."  "Fit- 
test" merely  means  those  best  adapted  to  live  and  reproduce 
in  the  environment  encountered.  This  process  of  natural  selec- 
tion does  not  cause  variations,  but  it  may  make  use  of  any 
variations  that  arise,  no  matter  what  their  source,  provided  they 


DOCTRINE  OF  EVOLUTION  AND  RELATED  IDEAS        497 

can  be  inherited.  Furthermore,  it  may  use  variations  of  small 
amount,  or  the  more  striking  variations  known  as  mutations. 
In  so  far  as  it  acts,  its  result  is  to  guide  evolution  into  suitable 
adaptation  to  the  conditions  that  are  able  in  any  degree  to  in- 
fluence life.  Nature  undoubtedly  can  exercise  a  veto  power  on 
the  direction  of  evolution. 

497.  Evolution  and  Man. — Many  people  are  deterred  from 
accepting  the  general  theory  of  evolution  because  of  unwilling- 
ness to  believe  that  man  and  his  high  mental  and  moral  qualities 
could  have  come  about  in  this  fashion.  It  is  of  course  impossible 
absolutely  to  prove  that  men  or  any  other  animals  have  thus 
evolved  from  the  lower  orders.  All  that  can  ever  be  done  is  to 
make  it  the  most  reasonable  explanation.  In  reality  it  cannot 
make  any  material  difference,  one  way  or  the  other,  whether 
man  developed  or  was  created  outright.  Just  as  much  power 
and  intelligence  are  necessary  in  the  evolution  of  man  as  in  out- 
right creation,  and  no  quality  of  man  is  any  less  valuable  or 
dignified  under  the  assumption  that  it  has  grown  than  under 
the  assumption  that  it  was  made  outright.  Man's  moral  place 
in  the  universe,  like  his  place  in  American  society,  is  properly 
determined  by  what  he  really  is  and  not  by  the  mere  incident  of 
the  method  of  becoming. 

498.    Topics  for  Library. — i.  Find  definitions  of  evolution.    Write  one  of 
your  own. 

2.  What  are  some  of  the  arguments  that  have  been  offered  to  support  the  view 
that  "acquired"  bodily  characters  are  transmissible? 

3.  State  some  arguments  that  have  been  advanced  against  the  view? 

4.  State  briefly  Galton's  law  of  heredity. 

5.  What  does  the  gradual  development  of  the  physical,  mental,  and  moral 
nature  of  the  human  individual  suggest? 

6.  State  DeVries'  doctrine  of  "mutation"  in  a  brief  way. 

7.  State  Johanssen's  "pure  line"  theory. 


CHAPTER  XXVII 

ECONOMIC  ZOOLOGY 

499.  Reference  has  already  been  made  in  the  appropriate 
places  in  the  text  to  some  of  the  most  important  ways  in  which 
the  various  branches  of  animals  are  related  to  human  welfare. 
We  are  now  in  a  position  to  look  back  over  the  economic  bearings 
of  animals  and  to  get  a  broader  view  of  the  part  they  play  in 
the  life  of  man.     This  is  a  point  at  which  Zoology  touches  the 
interest  of  every  citizen  of  the  world,  whether  he  has  any  other 
interest  in  it  or  not.     The  economic  aspect  should  be  one  of 
the  features  of  the  study  of  Zoology  in  our  schools. 

500.  An  Analysis  of  the  Ways  in  which  Animals  Affect  the 
Welfare  of  Man. — i.  Helpfully:  (a)  as  food;  (b)  for  clothing; 
(c)   in  the  saving  of  labor;   (d)   in  scientific  experiment  and 
medicine;  (e)  for  pleasure,  as  companions  and  pets;  (f)  in  minor 
miscellaneous  ways. 

2.  Hurtfully:  (a)  directly  hurtful  or  dangerous;  (b)  as  causes 
or  conveyors  of  disease  in  man;  (c)  as  enemies  of  our  friends 
among  the  animals;  (d)  as  destructive  to  vegetation;  (e)  as 
injurious  to  various  manufactured  products. 

These  headings  are  by  no  means  exhaustive  nor  of  equal 
merit,  but  they  will  give  the  student  some  conception  of  the 
closeness  of  the  bond  between  man  and  the  rest  of  the  animal 
kingdom. 

501.  Animals  as  a  Food  Supply. — The  difficulties  of  human 
life  have  been  such  at  various  places  and  times  that  man  has 
experimented  with  almost  all  kinds  of  food  that  have  held  out 
any  sort  of  promise  of  pleasure  or  nutrition.     Notwithstanding 
this,  very  few  of  the  invertebrates  have  been  given  a  wide  place 
in  his  bill-of-fare.     The  lowest  forms  eaten  are  the  "trepangs" 
(holothurians)  which  are  used  in  great  numbers  by  the  Chinese. 
In  the  branch  of  mollusks  the  oyster  is  at  the  head  in  importance. 

498 


ECONOMIC   ZOOLOGY  499 

There  are  also  a  number  of  marine  and  fresh-water  clams  and 
mussels  which  are  used  in  smaller  numbers.  Some  species  of 
gasteropods  (snails)  are  prized  in  certain  parts  of  the  world. 
It  is  believed  that  the  mussels  were  an  important  article  of  food 
to  some  tribes  of  North  American  Indians,  and  to  some  of  the 
peoples  of  the  old  world  when  they  were  in  a  corresponding  stage 
of  development.  Among  the  arthropods  the  lobster  comes  first 
in  importance,  although  it  has  been  fished  to  such  an  extent  that 
the  output  is  rapidly  diminishing  and  the  size  of  specimens  is 
steadily  decreasing.  Crayfish,  shrimps,  prawns,  and  crabs  are 
also  used  to  some  extent.  The  great  class  of  insects  furnishes 
only  the  honey  of  the  honey-bee. 

Every  class  of  vertebrates  furnishes  food  species.  The 
amphibians  and  the  reptiles  stand  lowest  in  this  respect.  The 
frogs  and  turtles  and  a  few  lizards  are  their  only  edible  repre- 
sentatives. The  fishes,  the  birds,  and  the  mammals  furnish 
the  staple  meat  foods.  A  very  large  per  cent,  of  the  fishes  are 
recognized  as  edible.  Many  species  of  the  sharks,  even,  are 
prized  although  they  are  actively  carnivorous,  and  the  car- 
nivorous animals  have  usually  not  been  regarded  as  good  to  eat. 
Many  species  of  birds  have  been  eaten,  but  those  that  rank  as  of 
real  importance  belong  chiefly  to  the  Gallinae,  the  Columbae,  and 
the  Anseres.  These  three  orders  of  birds  supply  to  man  hun- 
dreds of  millions  of  dollars  worth  of  his  choicest  food  annually. 

Among  the  mammals  many  species  have  been  eaten,  but 
the  great  division  of  ruminants  and  the  swine  furnish  the 
bulk  of  the  meat  food  now  used  by  the  human  race.  The 
horse  is  increasing  in  importance  in  this  respect.  Among 
uncivilized  people,  before  the  domestication  and  improvement 
of  the  ox,  the  sheep,  and  the  hog,  this  division  (ruminants) 
still  furnished  the  chief  wild  game  animals.  Besides  meat  they 
furnish  milk,  butter,  and  cheese.  In  the  far  north  the  walrus, 
the  bear,  and  the  reindeer  take  the  place  of  these  well-known 
forms. 

502.  Animals  as  a  Source  of  Clothing  for  Man. — In  the 

case  of  primitive  man  the  skins  and  furs  of  animals  were  the 
sole  source  of  clothing  and  one  of  the  means  of  making  tents 


50C  ZOOLOGY 

and  dwelling  places  comfortable  as  well.  As  the  art  of  weaving 
was  developed  and  the  discovery  and  perfecting  of  textiles 
derived  from  plants  made  headway,  we  became  less  dependent 
on  the  animals.  But  even  now,  in  one  form  or  another,  the 
skins  and  the  fur,  hair,  and  wool  of  animals  are  among  our 
choicest  clothing  materials.  The  mammals  are  of  course  the 
main  animal  source  of  clothing,  but  the  warmth  of  feathers  has 
long  been  recognized,  and  the  skins  and  feathers  of  birds  furnish 
articles  of  clothing  and  decoration.  The  important  mammals 
supplying  the  kind  of  hair  suitable  for  clothing  and  for  carpets 
and  other  coarser  fabrics  are:  the  various  species  of  sheep  and 
goats,  the  camels,  the  alpacas,  and  their  relatives.  Our  leathers 
are  made  from  the  skins  of  these  and  related  animals,  and 
from  some  of  the  carnivora.  Practically  all  the  ruminants 
produce  valuable  leathers.  Horse  hides  are  also  used  for  this 
purpose.  The  skins  of  many  of  the  soft,  thick-haired  animals 
are  dressed  with  the  hair  on,  and  are  known  as  furs.  Most  of 
these,  as  the  seals,  the  sable,  mink,  ermine,  weasel,  raccoon, 
foxes,  skunk,  members  of  the  cat  family,  and  some  others,  be- 
long to  the  carnivora.  A  few,  as  the  squirrel,  the  hare,  and  the 
beavers  (now  almost  extinct)  are  from  the  rodents. 

One  of  the  most  marvelously  delicate  and  beautiful  of 
our  fabrics,  silk,  is  a  secretion  sfc>un  out  by  the  larvae  of  the  silk- 
moth  in  making  the  cocoon  in  which  it  pupates,  or  changes  to 
the  adult  stage.  It  is  killed  by  being  put  into  hot  water,  and 
then  the  silk  thread  is  unwound.  The  silk  industry  is  of  much 
importance  in  China,  Japan,  Italy,  and  France. 

503.  Animals    Used    in    Saving    Human    Labor. — In    the 

earlier  stages  of  civilization  this  help  consisted  of  aid  in  hunting 
and  capture  of  food-animals.  The  dog  was  probably  one  of 
the  first  animals  domesticated.  Later  others  came  to  be  used 
for  riding,  for  drawing  loads  in  vehicles  or  otherwise,  for  plow- 
ing the  soil,  and  the  like.  The  camel,  the  ox,  the  horse,  and 
the  elephant  rank  among  the  most  valuable  in  these  respects. 
In  the  earlier  civilizations  of  the  orient,  the' camel  has  been  of 
the  utmost  value.  His  adjustment  to  the  tropical  and  semi- 
arid  conditions  is  striking.  The  "ship  of  the  desert"  is  an  older 


ECONOMIC    ZOOLOGY  501 

means  of  comrru'ive  than  the  navigation  of  the  sea.  His 
relatives  in  South  America,  the  llamas,  have  long  been  used  as 
the  chief  animal  of  burden  in  the  Andes.  The  reindeer  is  at 
once  camel,  horse,  and  ox  of  the  frigid  zone.  The  dog  is  also 
an  efficient  beast  of  burden  over  the  snows. 

In  modern  times  the  horse  has  r-eme  to  be  one  of  the  most 
valuable  of  animals  used  by  man,  .ranking  in  money  value  next 
to  the  cattle  in  the  United  States.  The  mule,  which  is  a  hybrid 
between  the  female  horse  and  the  male  ass  is  a  hardy,  strong, 
infertile  animal  and  one  of  our  most  valuable  beasts  of  burden 
in  agricultural  communities.  The  student  will  not  fail  to 
realize  that  more  powerful  groups  of  human  beings  have  used 
other  men  as  animals  in  very  much  the  manner  we  have  been 
describing.  While  slavery  is  not  legalized  over  much  of  the 
earth  it  is  still  known  in  other  forms. 

Steam  and  electricity  are  replacing  animal  power  at  many 
points,  but  there  is  little  likelihood  that  it  will  entirely  displace 
them  until  population  becomes  so  numerous  that  the  animals 
are  more  valuable  as  food  than  they  can  be  as  labor-saving 
devices. 

504.  Animals  in  Science  and  Medicine. — A  most  inter- 
esting way  in  which  animals  have  been  of  value  to  the  human 
race  grows  out  of  the  fact  of  the  general  likeness  between  man 
and  the  lower  animals.  It  is  safe  to  say  that  the  great  ad- 
vances in  surgery  which  have  accomplished  so  much  in  the 
saving  of  human  life  have  been  made  by  early  experimentation 
on  animals  in  the  laboratory,  even  more  than  by  the  actual 
direct  study  of  the  human  body  and  its  conditions.  Experi- 
ments quite  impossible  of  being  made  on  human  beings  have 
first  been  tried  on  animals  and  have  been  found  to  accomplish 
what  was  expected  of  them.  But  it  is  not  merely  in  surgery 
that  they  have  shown  themselves  most  valuable  to  human  life. 
In  all  the  work  on  antitoxins  and  serums  with  which  to  combat 
the  germ  diseases,  the  work  must  first  be  done  on  animals. 
The  antitoxin  of  diphtheria  comes  from  the  horse;  the  virus  by 
which  we  vaccinate  for  smallpox  is  obtained  by  giving  the 
disease  to  the  ox,  which  is  only  mildly  affected  by  it;  and  so  on 


502  ZOOLOGY 

through  a  considerable  list  of  such  diseases.  The  most  hopeful 
work  that  is  now  being  done  to  give  us  control  of  the  terrible 
disease  of  cancer  is  being  done  on  mice  and  rats,  otherwise  an 
enemy  to  human  interests. 

It  has  been  discovered  of  late  years  that  many  organs  of 
the  body  secrete  into  the  blood  that  passes  through  them, 
certain  substances  that  have  a  most  important  bearing  on  life. 
Sometimes  these  organs  become  diseased  and  refuse  to  do  their 
work.  This  may  be  true  of  the  digestive  glands,  of  the  thyroid 
glands  in  the  neck,  and  others.  In  such  cases  the  gastric  juice 
of  the  pig,  or  the  thyro-iodin  obtained  from  the  thyroid  of  the 
sheep,  or  pancreatin  obtained  from  other  animals  may  be  ad- 
ministered in  such  a  way  as  to  carry  on  the  function  or  even  to 
overcome  the  disease. 

Frogs,  rabbits,  guinea-pigs,  cats,  dogs,  sheep,  cows,  horses, 
and  monkeys  are  among  the  animals  that  have  been  most 
useful  in  these  respects. 

505.  Miscellaneous  Uses  of  Animals. — A  long  list  of  minor 
and  exceptional  uses  of  animals  could  be  mentioned.  The  dog 
and  cats  have  some  protective  value  about  the  homes  of  men, 
where  they  were  long  before  recorded  history  began.  They  have 
kept  homes  free'  of  vermin,  as  rats  and  mice  and  the  smaller 
animals,  and  the  dog  has  doubtless  been  of  protection  from 
other  enemies.  They  have  been  companions  for  childhood 
through  all  history. 

The  skins  and  tendons  and  bone  of  animals  figured  largely 
in  all  the  early  home  industries,  as  sewing  and  tying  and  the 
like.  In  modern  times  the  skins  are  used  in  many  ways,  as  rugs 
and  home  ornaments,  in  the  making  of  bags  and  other  receptacles. 
Horn,  bone,  and  ivory  from  the  teeth  and  tusks  of  animals  are 
largely  used  in  knife  handles,  piano  keys,  billiard  balls,  and 
numerous  other  objects.  Whale  oil  (sperm  oil)  has  long  been 
used  in  making  candles,  though  it  has  now  been  supplanted 
by  the  cheaper  mineral  oils.  Bristles,  brushes,  perfumes,  oils, 
glue,  buttons,  and  fertilizers  are  only  a  portion  of  the  numerous 
by-products  of  the  modern  meat-packing  houses.  Ornaments 
are  made  of  teeth,  of  corals,  and  of  pearls.  Many  animals  are 


ECONOMIC   ZOOLOGY  503 

scavengers,  attacking  and  destroying  carrion  and  other  decaying 
organic  matter. 

506.  Animals  Directly  Injurious  to  Man. — When  man  first 
appeared  on  the  earth  many  mammals  now  extinct,  much  larger 
and  fiercer  than  those  of  the  present  time,  were  abundant.     Un- 
questionably these  were  much  more  of  a  menace  to  him  then 
than  the  predaceous  animals  are  now.     These  animals,  except 
in  a  few  poorly  inhabited  parts  of  the  world,  are  now  practically 
negligible.     A  few  poisonous  snakes,  some  sharks  and  crocodiles, 
a  few  members  of  the  cat  and  dog  families  and  a  few  species  of 
bear,  almost  extinct,  about  exhaust  the  list  of  animals  really 
dangerous  because  of  size  or  ferocity.     But  in  place  of  these  there 
are  now  numerous  species  that  are  no  less  dangerous  to  him 
because  of  diseases  which  they  bring  to  man  directly  or  indirectly. 
Reference  has  been  made  to  Protozoa  which  produce  malaria 
and  yellow  fever,  and  to  the  intestinal  and  other  parasites  that 
belong  to  the  unsegmented  worms  and  produce  all  sorts  of  dis- 
comfort, inefficiency,  and  disease  among  men.     The  mosquitoes 
and  flies  and  other  insects  that  spread  these  diseases  are  just  as 
important  to  us  as  the  germs  themselves.     The  bubonic  plague 
is  believed  to  be  a  disease  of  rats  and  other  rodents  carried  to 
man  from  the  rat  by  fleas.     All  these  temporary  external  para- 
sites that  go  from  animal   to  animal,  as  lice,    fleas,  bedbugs, 
mosquitoes,  etc.,  are  especially  favorably  situated  to  be  the 
carrier  of  disease.     Cattle  are  subject  to  tuberculosis;  and  many 
investigators  believe  that  this  is  the  same  as  the  human  disease, 
or  at  least  that  it  is  intercommunicable  in  man  and  cattle. 

507.  Animals  Hurtful  to  our  Animal  Friends. — Our  domestic 
animals  are  apparently  almost  as  open  to  diseases  as  man 
himself.     Hence  it  is  that  there  are  many  animal  diseases,  caused 
by  the  various  parasites  of  blood  and  organs.     The  Texas  fever 
of  cattle;  the  haemoglobinuria  of  cattle  and  sheep;  pebrine,  a 
disease  of  the  silkworm;   the  sleeping  sickness  of  Africa;  etc., 
are  due  to  protozoa,  and  are  carried  by  ticks,  flies,  and  the  like. 
The  liver-rot  and  staggers  of  sheep,  and  the  various  tape- worm 
and  hook-worm  diseases  of  hogs,  dogs,   cattle,  and  man  are 
caused  bv  the  unsegmented  worms.     The  "bot"  diseases  of 


504  ZOOLOGY 

horses,  sheep,  and  cattle  are  produced  by  the  larval  stages  of 
flies.  Mange,  itch,  etc.,  as  found  in  various  domestic  animals, 
is  due  to  the  action  of  mites.  These  also  attack  poultry. 

508.  Animals  Hurtful  to  Plants  and  Plant  Products. — At 

this  point  animals,  more  particularly  the  insects,  do  great 
damage  to  man's  interests.  The  story  is  too  long  to  tell  here, 
but  insects  attack  plants  at  every  stage  of  their  history  from 
the  time  they  germinate  until  the  time  they  are  stored.  They 
devour  foliage,  fruits,  timbers,  seeds,  stored  grain,  and  manu- 
factured products.  It  is  said  that  "the  elm  has  eighty  species 
of  insects  that  are  more  or  less  supported  by  it ;  birches  and 
maples,  over  one  hundred;  corn  is  attacked  by  about  two 
hundred  species,  of  which  fifty  do  notable  injury  and  some 
twenty  are  pests;  apple  insects  number  some  four  hundred 
species." 

It  is  estimated  that  the  farmers  and  fruit  growers  of  America 
alone  lose  500,000,000  dollars  annually  from  the  ravages  of 
insects  on  crops  and  forests.  Man  has  done  much  to  control 
most  animals  except  the  insects,  and  something  to  control  them. 
The  most  effective  means  thus  far  found  is  by  spraying  plants, 
to  keep  away  the  females  when  about  to  lay,  or  to  kill  the  young; 
the  rotation  of  the  crops  in  such  a  way  that  the  insects  may  hatch 
without  finding  the  kind  of  food  on  which  they  depend ;  and  the 
introduction  of  animals,  either  predatory  or  parasitic,  that  attack 
the  hurtful  species. 

Occasionally  through  a  period  of  years  such  animals  will 
multiply  to  such  an  extent  as  to  become  a  plague,  when  in 
ordinary  years  they  merely  reduce  the  returns  of  the  agricultur- 
alist. Such  are  the  outbreaks  of  the  Rocky  Mountain  locust, 
the  Hessian  fly,  the  San  Jose  scale,  the  chinch  bug,  and  the  like. 
Less  frequently  similar  things  happen  in  respect  to  other  animals, 
as  the  plague  of  rabbits  in  Australia,  and  of  the  rats  in  the 
Bermudas. 

509.  Domestication  of  Animals. — Occasional  reference  has 
been  made  in  the  preceding  paragraphs  to  the  domestic  animals. 
This  means  something  quite  different  from  the  merely  useful 
animals.     This  term  refers  to  the  control  by  man  of  the  life 


ECONOMIC   ZOOLOGY  505 

and  habits  of  animals  to  such  degree  that  he  can  work  his  will 
with  them  almost  irrespective  of  the  natural  conditions.  This 
domestication  began  before  man  emerged  from  the  savage 
state — before  recorded  history  began.  The  dog  and  cat  were 
doubtless  among  the  earliest  forms,  and  the  camel,  the  horse, 
the  sheep,  the  donkey,  the  hog,  the  pigeon,  the  chicken,  all 
have  been  very  long  in  domestication. 

The  qualities  that  would  make  an  animal  suitable  for 
domestication,  and  which  doubtless  helped  to  determine  what 
forms  should  be  domesticated,  are:  some  gentleness  of  temper 
and  lack  of  extreme  nervousness,  capability  of  taking  training, 
ability  to  become  adapted  to  new  surroundings,  some  love  of 
locality,  and  usefulness,  for  food  or  for  some  other  of  man's 
real  or  fancied  needs.  The  effect  of  domestication  on  animals 
has  been  to  soften  and  dull  their  original  wild  instincts,  to 
render  them  less  active  and  alert,  to  render  them  dependent  on 
the  care  which  man  gives  them,  and  to  produce  a  tendency  to- 
take  on  fat  easily  when  food  is  plenty. 

510.  The  Animal  Industries. — The  animal  industries  clus- 
ter chiefly,  but  not  wholly,  about  the  domestic  animals.  These 
domestic  industries  relate  to  horses,  cattle,  mules,  sheep,  hogs, 
poultry  (including  chickens,  turkeys,  pigeons,  geese,  and  ducks), 
and  constitute  a  large  part  of  agriculture  in  the  United  States 
to-day.  The  successful  pursuit  of  these  industries  involves  a 
scientific  knowledge  of  the  animals  reared  and  of  their  possi- 
bilities and  demands.  Much  of  this  in  the  past  has  been  done 
in  a  haphazard  sort  of  way,  somewhat  incidental  to  the  raising 
of  crops.  In  the  last  half-century,  however,  there  has  been  a 
better  application  of  the  known  principles  of  stock-breeding 
and  of  selection  than  before.  The  modern  stock-breeder  crosses 
and  recrosses  various  strains  of  cattle  or  hogs  in  order  to  get 
variety  of  result.  Then  he  selects  what  pleases  him  most  and 
breeds  that  in  such  a  way  as  to  fix  and  increase  the  features 
which  he  regards  as  most  profitable,  whether  it  be  swiftness, 
size,  milk-giving,  cream-producing,  finer  wool,  fertility,  or  the 
power  to  take  on  fat.  This  is  the  reason  we  have  so  many 
varieties  of  all  the  animals  that  have  been  long  in  domes- 


506  ZOOLOGY 

ti cation.  Different  breeders  have  been,  more  or  less  consciously, 
selecting  different  qualities  in  animals.  Men  are  now  pushing 
this  process  even  further,  and  are  purposely  and  systematically 
producing  and  selecting  new  and  better  breeds.  The  evolution 
will  be  correspondingly  more  rapid. 

All  the  animal  culture  mentioned  above  is  done  in  con- 
nection with  plant  culture  and  often  on  the  best  land  the 
country  has.  There  are  other  industries  which  have  great 
possibilities  in  regions  not  suited  to  agriculture.  Man  has  used 
some  of  the  natural  resources  of  these  less  used  regions  in  a 
very  prodigal  and  reckless  way.  He  has  not  taken  pains  to 
look  ahead  and  to  act  scientifically,  simply  because  he  has  not 
been  compelled  to  do  so.  The  time,  however,  has  come  when 
the  farmer  uses  systematically  the  less  productive  parts  of  his 
farm  as  well  as  the  more  fertile.  If  not  fit  for  cultivation  he 
uses  it  for  sheep  or  goats  or  something  which  is  best  adapted 
to  it.  So  it  must  be  in  the  future  with  the  whole  earth.  The 
oceans,  the  rivers,  the  swamps,  the  ponds  and  lakes,  the  arid 
regions,  and  the  mountains  must  be  stocked  with  animals  that 
will  contribute  to  man's  food.  These  animals  must  be  helped 
to  take  the  place  of  those  which  are  of  no  use.  This  means  that 
the  various  fishing  industries,  as  mackerel,  cod,  herring;  and 
even  more  particularly  the  fresh-water  fisheries,  as  the  salmon, 
whitefish,  and  other  lake  and  river  fish,  shall  be  more  than  the 
mere  catching  and  canning  of  such  fish  as  have  been  able  to 
fight  their  own  way  through  to  successful  maturity;  they  will 
include  the  bending  of  all  possible  agencies  to  the  building  up, 
improving,  and  maintaining  the  numbers  and  quality  of  these 
fish.  Oyster,  lobster,  pearl-mussel,  and  sponge  fishing  will  be 
more  than  merely  to  put  on  the  market  the  biggest  possible 
amount  regardless  of  the  future;  it  will  rather  be  the  stocking  of 
all  suitable  places  of  the  ocean  margin  that  are  not  more  profit- 
ably used  for  something  else  with  the  young  of  the  types  wanted, 
keeping  away  the  enemies  of  the  species  as  well  as  possible,  and 
doing  artificially  all  that  can  be  done  to  insure  their  steady  and 
profitable  growth.  The  scientific  treatment  of  these  industries 
means  the  putting  on  the  market  only  those  that  have  got  their 


ECONOMIC   ZOOLOGY  507 

best  growth  and  with  the  least  injury  to  the  partly  grown  in- 
dividuals, just  as  it  does  on  the  farm  or  in  the  forest. 

The  conservation  of  our  natural  resources  also  means  that  the 
swamp  lands,  that  cannot  be  drained  and  used  more  profitably, 
and  the  shallow  streams  shall  be  made  to  produce  edible  frogs 
or  turtles  instead  of  the  other  inedible  amphibians  and  reptiles. 
Wherever  edible  fish  can  grow  pains  will  be  taken  to  place  them 
and  see  that  the  conditions  of  their  best  life  are  given  them. 

Various  bureaus  of  the  United  States  Government,  as  that 
of  Animal  Industry,  of  Fish  and  Fisheries,  of  Entomology,  and 
the  like;  the  Agricultural  Experiment  Stations  of  the  various 
States;  the  scientific  men  in  the  universities  and  colleges,  and 
many  practical  workers  who  are  in  these  fields  in  a  commercial 
way,  are  studying  the  problems  suggested  in  this  chapter  and 
are  trying  to  add  to  the  vital  resources  of  the  human  race  and 
its  increasing  population.  It  is  the  duty  of  every  American  to 
get  into  a  sympathetic  attitude  to  all  this  work. 

511.  Topics  for  the  Library. — i.  Give  a  report  of  the  United  States  Bureau  of 
Fish  and  Fisheries.  How  is  it  organized?  What  are  its  principal  stations? 
What  work  does  it  undertake  to  do?  What  are  its  chief  publications? 

2.  Discuss  similarly  the  Bureau  of  Animal  Industry;  the  Bureau  of  Entomology; 
the  Biological  Survey. 

3.  Give  an  account  of  one  of  the  inland  fish  hatcheries  and  its  work. 

4.  Why  are  the  vegetable  feeding  animals  preferred  for  food  above  the 
carnivora  ? 

5.  Why  are  woolen  fabrics  warmer  than  cotton  or  linen? 

6.  Will  the  vegetable  food  or  the  animal  food  that  can  be  produced  and  sup- 
ported on  ten  acres  of  ground  go  further  in  supporting  human  life  ?     Discuss 
carefully. 

7.  Do  you  think  the  tendency  will  be  to  increase  or  to  decrease  animal  culture 
as  population  begins  to  press  upon  the  resources  of  the  soil?     Give  the  biological 
reasons  fully. 

8.  What  are  the  chief  arguments  against  vivisection  and  experiments  on  living 
animals  by  investigators?     What  the  chief  arguments  in  favor  of  it? 

9.  Discuss  the  various  products  of  the  hog,  utilized  in  the  packing  houses. 

10.  Discuss  the  animals  that  frequent  human  habitations,  becoming  pests 
in  some  degree.     Give  the  habits  of  life  of  each  so  far  as  you  can  by  study  and 
reference. 

1 1 .  Discuss  with  some  care  what  is  known  of  the  history,  the  different  varieties, 
the  use,  etc.,  of  some  of  the  domestic  animals,  as  the  dog,  the  horse,  the  fowl,  the 
ox,  and  others. 

12.  Discuss  human  slavery  as  a  zoological  phenomenon.     To  what  is  due  the 
tendency  to  abolish  it? 


CHAPTER  XXVIII 

DEVELOPMENT  OF  ZOOLOGY 

512.  Introduction. — Long  before  man  discovered  a  way  to 
record  his  knowledge  he  must  have  learned  much  about  plants 
and  animals.     His  well-being  depended  on  it.     Here  he  got  his 
food.     His  most  dreaded  enemies -were  the  great  mammals  that 
were  on  the  earth  when  he  appeared.     His  success  in  evading 
his  enemies  and  in  capturing  his  food  made  it  necessary  that  he 
know  something  of  their  haunts  and  habits.     As  he  learned  to 
domesticate  some  of  these,  both  for  protection  and  for  food 
supply,   his  knowledge  was  extended.     As  the  animals  were 
slaughtered  for  food  and  clothing  and  for  his  primitive  religious 
rites,  he  learned  something  of  their  internal  structure.     This 
early  knowledge  probably  had  little  pure  scientific  interest  back 
of  it.     Most  of  it  was  incidental  to  the  uses  he  made  of  the 
animals,  doubtless  reinforced  somewhat  by  the  feeling  of  wonder 
and  the  esthetic  sense.     In  a  similar  way  human  accidents,  dis- 
eases, and  deaths  doubtless  led  to  some  study  of  the  human  body 
and  its  care  and  cure,  as  soon  as  human  intelligence  was  able  to 
grasp  the  situation. 

It  is  not  easy  for  us  to  forget  the  discoveries  of  the  last  400 
years  and  realize  how  fragmentary  and  unscientific  were  the 
ideas  of  the  early  times. 

513.  The  Greek  and  Roman  Periods. — So  far  as  we  need  con- 
sider here,  the  foundations  of  modern  zoology  were  laid  by  Aris- 
totle (384-322  B.C.).     It  must  not  be  imagined  however  that 
his  was  the  first  effort  to  bring  together  the  knowledge  already 
gained.     For   example,    ancient    Egyptian    documents   twelve 
centuries   older   than   Aristotle   give   elaborate   discussions   of 
medical  subjects. 

Aristotle  is  chiefly  known  as  a  philosopher  and  logician;  but 
he  was  a  most  prolific  student  in  many  realms.  He  made  two 
great  contributions  to  natural  history:  (i)  he  brought  together 

508 


DEVELOPMENT   OF   ZOOLOGY  509 

in  three  works — "Historic,  animalium,"  ilDe  partibus, "  and  "De 
generatione" — what  was  known  of  the  nature  and  classification 
of  animals,  their  anatomy,  and  their  development;  and  (2)  in 
his  own  great  discoveries  he  insisted  on  the  scientific  method  of 
depending  on  observed  facts  rather  than  on  tradition. 

He  did  the  first  task  so  well  that  his  successors  paid  little 
attention  to  his  scientific  point  of  view  for  nearly  2,000  years, 
but  merely  asked  what  Aristotle  had  said  on  the  subject. 

Another  Greek,  Galen,  did  for  medicine  very  much  what 
Aristotle  did  for  zoology.  He  made  anatomy  and  physiology 
the  foundation  of  medicine.  Because  it  was  forbidden  to 
dissect  the  human  body,  he  studied  the  bodies  of  monkeys 
and  other  mammals.  For  centuries  his  successors  followed  his 
conclusions  without  using  his  methods. 

The  name  of  the  Roman,  Pliny  (23-79  A-D0  is  usually  men- 
tioned here  because  of  a  great  collection  of  writings  on  natural 
phenomena.  He  fell  away,  however,  from  the  spirit  of  Aristotle 
and  Galen,  and  brought  together  without  much  discrimina- 
tion facts  and  nature-anecdotes  of  the  most  fanciful  kind.  The 
Romans  did  very  little  for  science. 

514.  The  Middle  Ages. — During  this  remarkable  period  of 
human  history    (400-1500   A.D.)   practically   no  progress   was 
made  in  the  natural  sciences.     It  was  a  period  of  waning  of 
the  old  human  civilizations  and  the  incubation  of  the  new. 
Wars,    difficulty   of   travel,    the   other-world   attitude   of    the 
Christian  church,  and  many  other  things  discouraged  observa- 
tion of  nature.     It  was  a  time  of  mysticism,  of  metaphysical 
speculation,   and  of  complete  dependence  on  the  authorities. 
A  large  portion  of  it  has  been  called,  not  inappropriately,  the 
Dark  Ages. 

515.  The  Modern  Period  (1500-1900)  and  its  Specializations. 

— In  every  department  of  human  interest  the  revival  following 
the  "dark  ages"  is  one  of  the  most  remarkable  movements  of 
history.  This  is  no  less  true  in  our  science  than  elsewhere.  It 
was  more  than  a  revival  of  interest.  The  authority  of  the 
church  and  of  the  past  had  so  intrenched  itself  that  the  greatest 
difficulty  lay  in  getting  back  freedom  of  investigation  and 


510  ZOOLOGY 

utterance.  The  first  task  was  to  throw  off  tradition  and 
authority.  The  names  of  Gallileo,  Descartes,  and  Vesalius  be- 
long to  this  period  of  revival.  Italy  had  a  large  part  in  the 
renewal  of  scientific  work,  as  in  other  fields. 

Starting  with  the  general  interest  in  natural  history  and 
classification  and  in  medicine  and  anatomy,  which  was  seen  in 
the  work  of  Aristotle  and  Galen,  the  modern  period  has  de- 
veloped a  great  many  special  departments,  as  the  various 
workers  have  followed  particular  studies.  We  may  profitably 
consider  the  progress  in  Biology  under  the  following  heads: 
(i)  Natural  history  and  classification;  (2)  Anatomy  and  its 
departments;  (3)  Physiology;  (4)  Embryology;  (5)  Philosophy 
of  Biology;  and  (6)  Applications  of  Biology.  Of  course  these 
divisions  of  interest  took  shape  only  gradually,  and  a  worker 
often  contributed  to  several  departments,  especially  at  the 
outset. 

516.  Natural  History  and  Systematic  Zoology. — Naturally 
enough  until  the  more  important  animals  had  been  recorded 
and  compared  and  classified,  and  their  more  striking  habits  and 
modes  of  living  studied  this  would  present  a  most  fruitful  and 
popular  field  of  investigation.  Gesner  (1516-1565),  a  Swiss 
physician,  studied  animals  widely  and  wrote  a  "history  of 
animals"  which  was  the  best  general  zoology  since  Aristotle. 
His  work,  which  was  profusely  illustrated,  largely  influenced 
later  studies. 

Ray  (1629-1705),  an  Englishman,  was  a  student  of  both 
plants  and  animals.  In  addition  to  his  own  original  studies  and 
discoveries  he  introduced  the  idea  of  the  species  as  a  definite 
group  of  organisms  arising  from  similar  parents.  He  was  thus 
the  real  founder  of  systematic  biology,  and  the  forerunner  of 
the  great  Swedish  naturalist  Linnaeus  (1707-1778),  who  pub- 
lished the  Sy sterna  Natures  in  an  effort  to  describe  all  the  known 
species  of  plants  and  animals.  Linnaeus  combined  Ray's  species 
with  the  genus,  and  first  used  the  generic  and  specific  names  as 
the  name  of  the  organism  (binominal  nomenclature).  He  in- 
vented also  the  brief  descriptions  of  genera  and  species.  His 
work  greatly  stimulated  systematic  studies  and  laid  the  founda- 
tion for  work  in  other  fields  of  zoology. 


DEVELOPMENT    OF   ZOOLOGY  511 

Buff  on  (1707-1788),  in  his  Natural  History,  Cuvier  (1769- 
1832)  in  his  "Animal  Kingdom,"  Lamarck  (1774-1829),  von 
Siebold  (1804-1885)  and  many  later  naturalists  have  worked 
to  perfect  our  system  of  classification  of  animals. 

517.  Anatomy  and  its  Divisions. — We  have  seen  that  Galen 
studied  the  anatomy  of  lower  mammals  to  throw  light  on 
medicine.  His  accounts  became  the  accepted  rule  for  centuries. 
Vesalius  (1514-1564)  a  physician,  born  in  Brussels,  made  elabo- 
rate studies  of  the  anatomy  of  the  human  body  as  well  as  of 
many  other  animals.  He  published  a  great  work  on  the  "Struc- 
ture of  the  Human  Body,"  in  which  he  denied  many  of  the  views 
held  previously.  His  discoveries  were  illustrated  by  many 
most  effective  figures  and  plates.  Both  because  of  the  scope  of 
his  work  and  the  investigative  spirit  Vesalius  laid  the  foundation 
of  modern  scientific  Biology.  It  was  not  so  much  that  all  his 
conclusions  were  correct.  He  did  what  was  much  better;  he 
made  it  possible  for  his  successors  to  criticise  and  add  to  his 
own  work.  This  makes  progress. 

From  the  study  of  the  habits  and  exterior  of  animals,  and 
the  study  of  human  anatomy,  students  would  naturally  pass  to 
a  study  of  internal  structure.  As  time  passed  and  details  of 
structure  accumulated,  two  lines  of  study  would  be  developed: 
a  comparison  of  the  structure  of  different  organisms,  and,  second, 
the  study  of  smaller  and  smaller  structures.  The  former  is 
called  Comparative  Anatomy,  and  the  latter  Histology  and 
Cytology. 

As  in  all  other  branches  there  were  forerunners,  but  the 
French  naturalist  Cuvier  was  the  first  zoologist  to  undertake  to 
compare  the  structures  of  all  the  groups  of  animals.  His  work 
strongly  influenced  his  successors,  particularly  in  France.  He 
was  thus  the  real  founder  of  comparative  anatomy.  It  was  on 
the  basis  of  these  studies  in  anatomy  that  he  made  his  con- 
tribution to  the  classification  of  animals.  Some  of  those  who 
continued  the  work  of  Cuvier  were,  Milne-Edwards  (1800-1885) 
a  Frenchman,  who  gave  us  the  idea  of  differentiation  of  parts  and 
division  of  labor  and  showed  its  value;  Richard  Owen  (1804- 
1892)  an  Englishman  who  developed  the  contrasts  of  analogy 


514  ZOOLOGY 

as  the  preformation  theory)  took  strong  hold  on  the  imagination 
of  the  students  of  the  subject,  and  the  physiologists  Haller, 
Bonnet,  Leibnitz,  etc.,  developed  the  idea  that  the  miniature 
embryo  preformed  in  the  egg  must  in  its  turn  contain  the  next 
generation  also  preformed  in  miniature,  and  this  the  next,  and 
so  on  indefinitely.  About  1677  it  was  discovered  that  the  sperm 
united  with  the  egg,  and  some  believed  that  the  miniatures  were 
in  the  sperm  rather  than  in  the  egg.  Kaspar  Wolff  in  1759 
attacked  the  whole  preformation  theory,  and  held  the  egg  to 
be  unorganized  at  first,  with  no  trace  of  the  future  organs  in  it. 
He  believed  some  vital  principle  organized  the  undifferentiated 
material  into  an  embryo.  Bitterly  opposed  at  first,  his  general 
view  came  to  dominate  Embryology. 

Von  Baer  (1792-1876)  is  recognized  as  the  greatest  of 
embryologists.  He  adopted  the  comparative  point  of  view 
in  embryology,  as  Cuvier  had  for  anatomy  and  Muller  had  for 
physiology  and  with  equal  fruitfulness.  Von  Baer  discovered 
that  all  the  higher  types  of  animals  develop  first  germ  layers 
(ectoderm,  entoderm,  and  mesoderm),  and  that  the  organs  are 
formed  by  the  growth  and  foldings  of  these. 

The  increasing  knowledge  of  cell  structure  and  behavior 
added  greatly  to  progress  in  embryology.  During  these 
years  it  became  clear  that  the  egg  and  the  sperm  were  single  cells, 
that  the  fertilized  ovum  divides  (cleavage)  as  ordinary  cells  do; 
that  the  descendants  of  these  cells  become  different  in  the 
different  layers,  tissues,  and  organs,  as  they  are  formed.  Gegen- 
bauer,  Koelliker,  Huxley,  Haeckel  and  many  others  made 
contributions  at  these  points. 

Building  on  the  preceding  work  Francis  Balfour  (1851-1882) 
of  Cambridge,  England,  published  in  1881  his  monumental 
"Comparative  Embryology."  It  made  available  the  best 
conclusions  of  the  past  and  developed  general  points  of  view 
that  had  only  slightly  or  not  at  all  appeared.  Many  great 
zoologists  have  given  large  attention  to  embryology  since  this 
time.  In  Germany,  Fol,  Oscar  Hertwig,  William  His,  Roux; 
and  in  America,  Brooks,  Minot,  Whitman,  Wilson,  Loeb,  and 
Morgan  have  made  brilliant  contributions  to  our  knowledge. 
These  include  the  study  of  the  processes  of  fertilization  and  the 


DEVELOPMENT   OF   ZOOLOGY  515 

behavior  of  the  cellular  elements  therein;  the  nature  of  the 
chromosomes  and  their  part  in  heredity;  the  effect  of  external 
agencies  upon  development;  the  origin  of  the  special  types  of 
tissues;  the  continuity  of  the  germ  plasm  and  even  of  germ  cells 
in  the  cycle  of  generations. 

520.  Philosophy  of  Biology. — In  the  early  periods  of  human 
thinking  the  theories  outran  the  facts.  During  all  the  time 
much  philosophy  has  accompanied  the  discoveries.  So  all  the 
great  names  that  have  been  mentioned  have  contributed  some- 
thing to  our  philosophy  of  Biology.  Important  as  the  field 
is  we  can  here  only  mention  a  few  salient  steps.  Some  of 
them  have  already  been  suggested.  The  nature  of  some  of 
them  have  been  more  completely  discussed  in  Ch.  XXVI.  These 
problems  include  the  origin  and  nature  of  living  things,  the 
causes  that  have  operated  to  bring  them  to  their  present  state, 
and  the  principles  that  seem  to  underlie  the  process. 

It  may  be  said  that  two  main  theories  obtained  among  the 
ancients  to  explain  life  as  we  have  it  on  the  earth.  One  held 
that  organisms  were  created  supernaturally  much  as  they  are 
at  present,  closely  suited  to  the  environment  in  which  they 
live.  The  other  considered  that  in  some  way  the  present  is  a 
natural  development  from  the  past,  and  that  organisms  have 
grown  into  their  various  adjustments  to  external  conditions. 
It  is  impossible  to  say  how  early  this  latter  idea  of  evolution,  so 
generally  held  at  present,  originated.  We  find  evidences  of  it 
before  Aristotle  and  still  more  in  his  writings. 

Theoretically,  the  following  views  of  life  might  be  held : 

1.  That  life  has  always  been  on  the  earth,  and  only  life 
can  give  rise  to  life. 

2 .  That  at  some  time  life  appeared  on  the  earth,  from  non-life, 

a.  Suddenly,  much  as  it  is  at  present. 

b.  Gradually,    starting   in    simple   form    and    becoming 
complex  as  at  present. 

Two  fundamental  questions  are  here :  Did  life  start  de  now  at 
some  time?  Was  it  constant  after  it  started?  Another  has 
arisen  since:  Can  life  start  anew  at  present  (spontaneous 
generation)  ? 


514  ZOOLOGY 

as  the  preform ation  theory)  took  strong  hold  on  the  imagination 
of  the  students  of  the  subject,  and  the  physiologists  Haller, 
Bonnet,  Leibnitz,  etc.,  developed  the  idea  that  the  miniature 
embryo  preformed  in  the  egg  must  in  its  turn  contain  the  next 
generation  also  preformed  in  miniature,  and  this  the  next,  and 
so  on  indefinitely.  About  1677  it  was  discovered  that  the  sperm 
united  with  the  egg,  and  some  believed  that  the  miniatures  were 
in  the  sperm  rather  than  in  the  egg.  Kaspar  Wolff  in  1759 
attacked  the  whole  preformation  theory,  and  held  the  egg  to 
be  unorganized  at  first,  with  no  trace  of  the  future  organs  in  it. 
He  believed  some  vital  principle  organized  the  undiflerentiated 
material  into  an  embryo.  Bitterly  opposed  at  first,  his  general 
view  came  to  dominate  Embryology. 

Von  Baer  (1792-1876)  is  recognized  as  the  greatest  of 
embryologists.  He  adopted  the  comparative  point  of  view 
in  embryology,  as  Cuvier  had  for  anatomy  and  Muller  had  for 
physiology  and  with  equal  fruitfulness.  Von  Baer  discovered 
that  all  the  higher  types  of  animals  develop  first  germ  layers 
(ectoderm,  entoderm,  and  mesoderm),  and  that  the  organs  are 
formed  by  the  growth  and  foldings  of  these. 

The  increasing  knowledge  of  cell  structure  and  behavior 
added  greatly  to  progress  in  embryology.  During  these 
years  it  became  clear  that  the  egg  and  the  sperm  were  single  cells, 
that  the  fertilized  ovum  divides  (cleavage)  as  ordinary  cells  do; 
that  the  descendants  of  these  cells  become  different  in  the 
different  layers,  tissues,  and  organs,  as  they  are  formed.  Gegen- 
bauer,  Koelliker,  Huxley,  Haeckel  and  many  others  made 
contributions  at  these  points. 

Building  on  the  preceding  work  Francis  Balfour  (1851-1882) 
of  Cambridge,  England,  published  in  1881  his  monumental 
"Comparative  Embryology."  It  made  available  the  best 
conclusions  of  the  past  and  developed  general  points  of  view 
that  had  only  slightly  or  not  at  all  appeared.  Many  great 
zoologists  have  given  large  attention  to  embryology  since  this 
time.  In  Germany,  Fol,  Oscar  Hertwig,  William  His,  Roux; 
and  in  America,  Brooks,  Minot,  Whitman,  Wilson,  Loeb,  and 
Morgan  have  made  brilliant  contributions  to  our  knowledge. 
These  include  the  study  of  the  processes  of  fertilization  and  the 


DEVELOPMENT   OF   ZOOLOGY  515 

behavior  of  the  cellular  elements  therein;  the  nature  of  the 
chromosomes  and  their  part  in  heredity;  the  effect  of  external 
agencies  upon  development;  the  origin  of  the  special  types  of 
tissues ;  the  continuity  of  the  germ  plasm  and  even  of  germ  cells 
in  the  cycle  of  generations. 

520.  Philosophy  of  Biology. — In  the  early  periods  of  human 
thinking  the  theories  outran  the  facts.  During  all  the  time 
much  philosophy  has  accompanied  the  discoveries.  So  all  the 
great  names  that  have  been  mentioned  have  contributed  some- 
thing to  our  philosophy  of  Biology.  Important  as  the  field 
is  we  can  here  only  mention  a  few  salient  steps.  Some  of 
them  have  already  been  suggested.  The  nature  of  some  of 
them  have  been  more  completely  discussed  in  Ch.  XXVI.  These 
problems  include  the  origin  and  nature  of  living  things,  the 
causes  that  have  operated  to  bring  them  to  their  present  state, 
and  the  principles  that  seem  to  underlie  the  process. 

It  may  be  said  that  two  main  theories  obtained  among  the 
ancients  to  explain  life  as  we  have  it  on  the  earth.  One  held 
that  organisms  were  created  supernaturally  much  as  they  are 
at  present,  closely  suited  to  the  environment  in  which  they 
live.  The  other  considered  that  in  some  way  the  present  is  a 
natural  development  from  the  past,  and  that  organisms  have 
grown  into  their  various  adjustments  to  external  conditions. 
It  is  impossible  to  say  how  early  this  latter  idea  of  evolution,  so 
generally  held  at  present,  originated.  We  find  evidences  of  it 
before  Aristotle  and  still  more  in  his  writings. 

Theoretically,  the  following  views  of  life  might  be  held : 

1.  That  life  has  always  been  on  the  earth,  and  only  life 
can  give  rise  to  life. 

2 .  That  at  some  time  life  appeared  on  the  earth,  from  non-life, 

a.  Suddenly,  much  as  it  is  at  present. 

b.  Gradually,    starting   in    simple   form    and    becoming 
complex  as  at  present. 

Two  fundamental  questions  are  here :  Did  life  start  de  novo  at 
some  time?  Was  it  constant  after  it  started?  Another  has 
arisen  since:  Can  life  start  anew  at  present  (spontaneous 
generation)  ? 


516  ZOOLOGY 

John  Ray  about  1725  crystallized  the  idea  held  by  Linnaeus 
and  many  of  his  predecessors  that  organisms  arose  de  novo,  by 
special  creation,  and  since  that  time  have  remained  practically 
constant.  Cuvier  gave  this  view  the  support  of  his  influence. 
Lamarck  (1774—1829)  contended  that  species  were  changeable, 
that  the  fossils  in  the  strata  were  remains  of  extinct  species 
which  were  the  ancestors  of  the  present  ones.  He  held  that  the 
needs  of  the  organism  resulting  in  the  use  and  disuse  of  organs, 
and  the  action  of  the  environment  upon  the  organism  changed 
individuals  and  that  these  modifications  were  transmitted  in 
some  measure  to  the  next  generation.  Thus  he  thought  evolu- 
tion occurred.  He  was  the  founder  of  the  evolution  theory  in 
its  modern  sense. 

Charles  Darwin  (1809-1882),  accepting  essentially  La- 
marck's views,  added  to  them  the  principle  of  natural  selection 
(§137)  through  the  struggle  for  existence  and  the  elimination 
of  the  unfit.  This  was  his  great  contribution.  It  was  so  rea- 
sonable and  he  supported  it  with  such  an  array  of  facts  that  it 
put  the  evolution  idea  on  a  firm  footing  and  stimulated  bio- 
logical thought  and  investigation  more  perhaps  than  any  other 
suggestion  ever  proposed.  A.  R.  Wallace  shares  with  Darwin 
the  honor  of  discovering  the  idea  of  natural  selection,  although 
the  latter 's  statement  was  theoretical  rather  than  experimental. 

With  the  work  of  Darwin  the  idea  of  the  variability  of  species 
may  be  said  to  become  established.  The  questions  now  hinged 
upon  the  method  of  origin  of  the  variations  and  their  trans- 
missibility  to  new  generations. 

August  Weismann  (1834-1915)  held  that  body  characters 
gained  as  Lamarck  suggested  by  use  and  disuse  of  the  body, 
could  not  be  transmitted  to  the  next  generation.  Only  those 
changes  which  take  place  in  the  germ  plasm  could  be  inherited. 
He  advanced  the  idea  of  a  continuous  germ  plasm  from  which 
body  after  body  arises  in  successive  generations.  Weismann 
held  therefore  that  Lamarck's  supposed  factors  could  have 
nothing  to  do  with  evolution.  Their  influence  stops  with  the 
body  of  the  individual.  He  gave  more  value  to  natural  selection 
than  even  Darwin. 

Gregor  Mendel   (1822-1884)   through  his  breeding  experi- 


DEVELOPMENT   OF   ZOOLOGY  517 

ments  produced  facts  which  strongly  support  Weisman's  idea 
of  continuity  of  germ  plasm. 

DeVries  and  Johanssen,  living  biologists,  have  strongly 
called  in  question  Darwin's  explanation  by  showing  that  the 
small  fluctuations  (continuous  variations)  of  body  which  Darwin 
emphasized  are  not  disposed  to  accumulate  by  selection.  De- 
Vries believes  that  only  "mutations"  (discontinuous  variations) 
are  subject  to  inheritance.  There  is  however  probably  no  neces- 
sary conflict  between  the  discoveries  of  Darwin,  Weismann, 
Mendel,  DeVries,  Johanssen.  When  all  the  facts  are  known  we 
shall  probably  find  that  all  of  them  have  found  some  truth, 
and  that  none  of  them  is  as  potent  as  its  discoverer  imagined. 

521.  Applications  of  Biology. — From  the  earliest  days  when 
Biology  was  a  mere  appendage  to  medicine  and  of  the  act  of 
living,  it  has  been  making  continuous  contributions  to  human 
welfare.  It  has  given  us  more  knowledge  of  the  health  and 
working  of  our  own  body;  it  has  made  possible  the  wonders  of 
surgery;  it  has  enabled  us  to  master  many  of  the  contagious 
and  infectious  diseases  of  men  and  animals ;  it  has  taught  us  how 
to  culture,  breed,  and  select  plants  and  animals  for  our  uses, 
and  to  prepare  their  products  to  best  advantage.  A  very  large 
proportion  of  human  industry  depends  directly  upon  Biology. 

Some  of  the  steps  in  the  progress  of  medicine  have  been 
outlined  heretofore.  Many  of  the  early  biologists  we  have  seen 
to  be  physicians. 

Leeuwenhoek,  one  of  the  early  users  of  the  microscope,  dis- 
covered bacteria  in  1687.  Soon  after  this  it  was  first  suggested 
that  microscopic  forms  might  have  something  to  do  with  con- 
tagious diseases.  In  1840,  Heale  took  the  position  that  all  such 
diseases  are  caused  by  germs. 

Louis  Pasteur  (1822-1895),  a  Frenchman,  was  one  of  the 
most  remarkable  scientists  of  all  time.  He  discovered  that 
fermentation  is  due  to  microscopic  organisms  (1857) ;  established 
the  actual  connection  between  certain  germs  -and  the  diseases 
produced  by  them;  showed  that  germs  could  be  made  less 
harmful  by  being  grown  in  certain  conditions,  and  that  these 
attenuated  germs  would  produce  only  a  mild  form  of  the 


518  ZOOLOGY 

disease  which  would,  however,  give  immunity  just  as  fully  as 
the  severer  forms.  He  succeeded  in  inoculating  against  fowl 
cholera,  splenic  fever  of  cattle,  and  finally  against  hydro- 
phobia. In  the  institute  which  bears  his  name  similar  principles 
have  been  extended  to  bubonic  plague,  lockjaw,  and  other 
diseases.  Here  also  was  discovered  the  antitoxin  for  diphtheria. 
Robert  Koch,  born  in  1843,  is  the  Pasteur  of  Germany.  He 
discovered  and  isolated  the  germ  of  tuberculosis  (1881)  and  of 
Asiatic  cholera  (1883).  Sir  Joseph  Lister  the  great  English 
surgeon,  born  in  1827,  using  Pasteur's  discoveries  first  applied 
antiseptics  to  wounds  in  order  to  destroy  the  germs  that  might 
enter.  This  was  the  foundation  of  aseptic  surgery  which  under- 
takes so  to  sterilize  everything  brought  near  an  operation  that 
germs  shall  not  enter  at  all. 

One  of  the  most  interesting  things  to  hold  in  mind  about 
biological  discoveries  is  this:  it  is  impossible  to  tell  how  impor- 
tant to  human  welfare  any  discovery  may  prove  to  be.  No  one 
could  have  imagined  that  these  students  with  the  microscope 
studying  the  organisms  of  decomposition  would  be  led  gradually 
to  the  most  profoundly  important  facts  bearing  on  human  life. 
It  was  not  quite  200  years  from  the  discovery  of  bacteria  until 
Pasteur  had  discovered  a  way  to  overcome  some  of  the  diseases 
produced  by  them. 

Other  fields  in  which  the  discoveries  of  the  students  of 
biology  have  been  most  valuable  in  practical  life  are :  the  breed- 
ing and  improvement  of  our  food  and  forage  plants;  the  im- 
provements of  the  domestic  animals;  the  encouragement  of 
wild  food  animals,  as  oysters  and  fish;  the  preservation  of 
foods;  conservation  of  forests;  the  holding  in  check  the  pests, 
largely  insect,  that  often  threaten  to  exterminate  domestic 
plants  and  animals.  In  these  fields  great  progress  will  continue. 

With  our  increased  knowledge  of  heredity  and  breeding, 
scientists  are  asking  the  question  whether  these  principles  which 
have  done  so  much  to  improve  the  races  of  plants  and  animals 
may  not  be  used  to  secure  more  rapid  progress  in  man.  Pearson, 
Bateson,  Davenport  and  others  claim  perfectly  soundly  that 
many  imperfections,  such  as  imbecility,  epilepsy,  criminality, 
insanity,  and  other  inheritable  weaknesses,  are  bred  back  into 


DEVELOPMENT   OF   ZOOLOGY  519 

the  blood  of  the  race  every  generation  by  unsound  individuals. 
This  process  in  our  herds  would  produce  scrubs.  It  is  believed 
that  the  race  owes  itself  the  duty  not  merely  to  care  for  the  in- 
dividual after  he  is  born,  but  to  strive  to  see  that  every  individual 
shall  be  as  well  born  as  possible.  This  science  of  human  breeding 
is  called  Eugenics. 


CHAPTER  XXIX 

EXERCISES  IN  COMPARATIVE  PHYSIOLOGY,   MORPHOLOGY,  AND 

ECOLOGY 

522.  Now  that  the  student  has  studied  in  some  detail  the 
work  which  even  the  simplest  organisms  must  perform,  the 
organs  by  means  of  which  this  necessary  work  is  done  in  some 
of  the  principal  types,  and  the  relations  which  animals  assume 
to  each  other  and  to  the  environment  in  general,  it  is  desirable 
that  he  should  bring  these  facts  into  such  relations  that  they 
may  be  compared.  The  likenesses,  the  unlikenesses,  and  the 
progressive  differentiation  are  thus  brought  into  clear  relief. 
The  following  outline  exercises  are  intended  to  guide  the  stu- 
dent in  this  task.  They  are  by  no  means  exhaustive,  but  will 
suggest  the  principal  points  most  essential  to  such  a  resume. 
The  laboratory  notes,  the  textbook,  and  all  the  reference  books 
at  his  command,  should  be  used  by  the  student.  The  student 
should  be  able  to  cite  his  authority  for  all  important  statements 
not  his  own,  and,  if  possible,  corroborate  by  reference  to  more 
than  one  authority.  Tables  with  parallel  columns  such  as 
those  on  pages  343  and  346  furnish  an  economical  and  other- 
wise satisfactory  mode  of  displaying  the  results  of  these  studies. 

I.  Fundamental  Form   (Promorphology). — Indicate  for  each 
of  the  important  phyla,  or  for  chosen  representatives  of  them, 
the  following  matters  of  general  form:  kind  of  symmetry  rep- 
resented and  the  perfection  of  its  development;  the  degree  and 
character   of   segmentation;   the  position,  number,    character, 
and  arrangement  of  the  appendages;  the  external  and  the  in- 
ternal evidences  of  cephalization ;  the  relation  of  the  principal 
organs  of  the  animal  to  the  horizontal  and  vertical  planes. 

II.  Physiology    and    Morphology. — Compare    the    mode    of 
performing  the  following  functions  and  the  organs  used  therein, 
in  all  the  principal  animal  phyla. 

1.  The  capture  of  food:  the  method;  the  organs  devoted  to 
it ;  and  the  relation  of  these  to  the  nature  of  the  food  used. 

2.  Digestion;  physical  and  chemical. 

520 


EXERCISES  521 

3.  Circulation,  as  pertaining  both  to  the  nature  of  circulat- 
ing fluid  and  to  the  organs  moving  it;  the  relation  of  the  whole 
process  to  the  organs  and  function  of  digestion  and  respira- 
tion in  the  types  chosen. 

4.  Respiration:  the  medium  containing  the  oxygen,  and  the 
contrivances  for  securing  it. 

5.  Excretion:  note  and  classify  the  chief  modes  of  elimin- 
ating waste  materials  observed  in  the  animal  phyla. 

6.  The  body  cavity  (ccelom)  in  relation  to  digestion,  cir- 
culation and  excretion. 

7.  Physical   support   and  protection    (skeletal   structures); 
their  position,  structure,  and  mode  of  formation. 

8.  Motion  and  locomotion:  degree  of  each;  relation  of  the 
muscular  or  contractile  elements  to  the  skeletal.     The  medium 
used  in  locomotion;  the  principal  special  devices  in  each  group 
for  the  solution  of  the  problems  presented  by  the  medium. 

9.  Sensitiveness:  the  kinds  of  stimuli  to  which  the  organ- 
isms in  the  various  groups  react;  the  differences  in  the  differ- 
ent phyla  in  each  of  the  various  classes  of  sense  organs,  as  to 
structure,  position,  and  manner  of  action;  the  number,  position 
and  perfection  of  the  nerve  centres;  and  the  relation  of  the 
nerve  centres  to  the  sense  organs  and  to  the  muscles. 

10.  Reproduction.     The  various  methods,  and  the  special 
ends  accomplished  by  each;  rate;  number  of  offspring;  paren- 
tal care;  sex  dimorphism;  alternation  of  generation;  partheno- 
genesis. 

III.  Ecology   and   Adaptations   to   the  Environment. — Com- 
pare the  animal  groups  from  the  following  points  of  view. 

1.  General  habitat:  aquatic,  fresh  or  salt  water;  terrestrial; 
aerial. 

2.  Migration  or  other  special  means  of  effecting  distribution 
from  the  point  of  origin. 

3.  Degree  of  connection,  organic  or  social,  between  the  in- 
dividuals of  a  species;  gregarious,  social  and  communal  life; 
resulting  social   qualities;   degree  of  division  of  labor;  poly- 
morphism. 

4.  Power  of  regenerating  lost  parts. 

5.  Growth;    rate,    and    ultimate    size;    longevity.     Special 
hindrances  to  growth. 


522  ZOOLOGY 

6.  Relation  to  human. welfare:  use  as  food;  effects  on  crops 
and  domestic  animals;  the  production  or  dissemination  of  dis- 
ease in  man;  capability  of  domestication;  other  qualities  help- 
ful or  hurtful  to  human  interests.     Which  phyla  furnish  spe- 
cies susceptible  of  domestication  ? 

7.  Diseases  among  animals  other  than  man. 

8.  Coloration:  pigments,  internal  and  external;  other  modes 
of  producing  color;  location  of  the  color;  supposed  uses. 

9.  Principal  methods  of  avoiding  or  surviving  unfavorable 
periods,  as  cold,  drouth,  and  the  like. 

10.  Qualities  of  offense  and  defense. 

11.  Protective  resemblance   and   mimicry.     Other   passive 
modes  of  protection. 

12.  Parasitism    and    the   degree   of    degeneracy    resulting 
from  it. 

IV.  Geographical  Distribution. — Select  several  representa- 
tive species  from  each  animal  phylum  and  learn  everything 
you  can  concerning  their  distribution  on  the  earth.  Are  they 
local  species  or  cosmopolitan  species?  What  seems  to  be  the 
reason  for  the  fact?  Are  all  the  phyla  cosmopolitan?  Com- 
pare the  animal  phyla  from  the  following  points  of  view: 

1.  The    facilities    for    migration.     The    special    modes    of 
migration,  both  active  and  passive. 

2.  What  are  the  principal  barriers  to  migration  and  distri- 
bution in  the  case  of  the  representatives  chosen  for  study  ? 

3.'  Find  instances  of  species  of  animals  apparently  closely 
related,  with  different  geographical  distribution.  Compare, 
for  example,  the  species  of  hares  and  rabbits  found  in  North 
America;  the  species  of  lynx;  of  bears;  of  the  alligators;  spe- 
cies of  Unio;  of  the  lobster;  of  the  genus  Equus. 

4.  Make  a  local  map  of  the  region  about  your  school  on  a 
large  scale.  Show  all  ponds,  streams,  lakes,  marshes,  mead- 
ows, uplands,  forests,  etc.  Show  by  suitable  symbols  where 
various  species  of  animals  are  to  be  found  with  reasonable 
certainty.  Keep  in  a  note-book  belonging  to  the  laboratory 
a  memorandum  of  each  new  species  found,  and  of  a  new  local- 
ity for  a  known  species.  In  time  the  map  and  the  note-book 
will  be  a  good  account  of  the  local  distribution  of  species. 


APPENDIX 

LABORATORY  SUGGESTIONS 

1.  The  Relation  of  the  Descriptive  Work  to  that  of  the  Laboratory  and  Field. 

—If  time  were  of  no  consideration,  it  would  perhaps  be  desirable  for  each  student 
to  get  all  his  information  concerning  animals  at  first  hand.  Even  under  this 
most  favorable  assumption,  however,  his  information  would  have  a  detached  and 
unrelated  quality  which  can  only  be  corrected  by  lectures  or  textbook.  This 
indicates  the  author's  view  of  the  purpose  of  the  body  of  the  text.  It  is  to  con- 
serve the  pupil's  time  and  to  unify  his  own  necessarily  scattered  observations  in 
such  a  way  as  to  give  them  a  vital  and  permanent  interest.  For  this  end  the 
practical  work  in  each  phylum  of  animals  should  precede  the  descriptive  and  not 
be  used  merely  to  illustrate  it.  The  textbook  instruction  and  library  references 
should  have  a  much  wider  scope  and  fuller  illustrative  detail  than  is  possible 
in  the  laboratory. 

2.  The  Nature  of  the  Practical  Work. — Personally  the  author  has  little  sym- 
pathy with  the  sentiment,  so  much  in  evidence  of  recent  years,  that  the  most  bizarre 
and  superficially  interesting  phenomena  are  the  ones  most  likely  to  lead  to  good 
educational  results.     These  may  be  well  enough  in  their  place,  but  their  best 
possible  place  when  not  abused  is  only  to  heighten  interest  in  the  more  important 
relations  and  phenomena  of  animal  life.     The  animal  furnishes  ^nteresting  and 
important  facts  in  two  essential  relations:  (i)  the  internal,  in  connection  with  which 
we  are  concerned  equally  with  the  fundamental  structure  and  with  its  relation  to 
the  work  to  be  done  by  the  organism;  and  (2)  the  external,  in  which  we  are  in- 
terested in  this  same  work  done  by  the  parts  of  the  organism,  but  in  relation  to 
the  conditions  on  the  outside  of  the  animal.     Physiology  is  thus  the  connecting 
link  between  morphology  and  ecology.     The  exercises  of  this  book  have  been 
arranged  in  the  main  to  lead  the  student  to  see  first  what  the  animal  types  do; 
secondly,  the  relation  of  this  activity  to  the  outside  world;  and  thirdly,  the  more 
important  structures  by  which  this  relation  is  maintained.     The  practical  work 
should  then  be  (i)  physiological,  which  involves  both  the  field  and  the  laboratory; 
(2)  ecological,  chiefly  in  the  field;  and  (3)  morphological,  chiefly  in  the  laboratory. 
In  each  case  the  student  should  be  caused  to  take  the  attitude  of  answering 
questions,  preferably  of  his  own  asking,  rather  than  of  verifying  descriptions. 
The  laboratory  outlines  seek  to  raise  questions  rather  than  to  supply  answers,  and 
to  suggest  topics  of  value  rather  than  to  exhaust  the  subject.     The  best  possible  outline 
is  that  which  pupil  and  teacher  construct  together. 

3.  The  Order  of  Work  and  the  Time  to  be  Given  (see  table). — The  author  has 
arranged  the  matter  in  the  book  as  it  appears  to  him  it  should  be  presented  if 
the  various  organisms  were  always  available  when  needed,  a  condition  which 
every  teacher  knows  to  be  contrary  to  fact.     Everything  considered,  the  author 
thinks  the'best  results  may  be  had  by  beginning  the  year's  work  in  the  spring  term 
and  finishing  it  in  the  autum  n  term  of  the  next  year.     No  arrangement  of  courses 

523 


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ZOOLOGY 


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LABORATORY    SUGGESTIONS  525 

can  be  best  for  all,  but  the  following  tables  may  be  suggestive  as  to  the  order  of 
treatment,  time  to  be  devoted  to  various  types,  and  the  like.  "Practical"  is 
meant  to  include  field  work,  laboratory  work,  demonstrations,  and  themes  worked 
out  in  the  library.  A  whole  year's  work  is  supposed  to  embrace  not  less  than 
three  class-room  periods  and  two  double  laboratory  periods  a  week  for  about 
thirty-six  weeks.  The  author  has  purposely  placed  at  the  disposal  of  the  teacher  in 
this  textbook  about  twice  as  much  work  as  can  be  done  well  in  the  allotted  time. 
The  purpose  of  this  is  that  each  teacher  may  have  the  privilege  of  electing  material 
most  suited  to  the  special  circumstances,  and  yet  have  before  him  an  ideal  of 
what  a  thorough  elementary  course  should  cover. 

For  a  course  covering  one-half  year  and  given  in  the  spring  term  the  order 
would  be  about  that  of  I  and  the  time  about  as  in  III.  In  a  course  of  one-half 
year  the  bulk  of  the  matter  in  fine  print  should  be  omitted,  or  used  in  just  such 
measure  as  time  will  permit.  The  marine  forms,  which  the  majority  of  schools 
will  have  to  study  from  preserved  materials,  and  the  general  part  of  the  text 
(Chapters  I  to  VIII)  should  be  studied  in  the  winter  months  when  the  local  animals 
are  least  active.  In  connection  with  the  review  in  Chapter  XXIX,  Chapters  I 
to  VIII  should  be  reread  by  the  student.  Such  a  review  will  be  especially  helpful 
after  the  student  has  a  larger  body  of  zoological  details  at  his  command.  Chap- 
ters XXV  to  XXVIII  will  strengthen  the  general  impressions  gained  in  the 
earlier  parts. 

4.  The  Laboratory  and  its  Equipment. — (a)  The  laboratory  or  work  room 
should  be  well  lighted,  and  supplied  with  flat-topped  tables;  the  plainer,  the 
better.  These  should  be  29  to  30  inches  in  height.  If  possible  each  student 
should  have  a  drawer  where  he  may  keep  his  instruments  and  records.  Sinks 
with  running  water  are  of  course  very  desirable.  Slop-jars  of  earthenware  should 
be  provided  for  refuse  dissections,  and  the  like. 

(6)  There  should  also  be  another  room  in  which  living  animals  may  be  kept. 
Very  often  a  part  of  the  basement  with  south  exposure  may  be  utilized  for  this 
purpose.  The  temperature  should  not  fall  to  the  freezing  point,  nor  rise  unduly 
when  the  furnace  is  heated.  In  such  a  room  as  this  many  animals  may  be  kept 
much  beyond  the  period  when  they  disappear  outside.  Fruit  jars,  tumblers, 
shallow  glass  or  crockery  dishes,  and,  best  of  all,  battery -jars  of  various  sizes  should 
be  accumulated  here.  With  a  little  ingenuity  aquarium  vessels  of  good  size,  with 
glass  sides,  may  be  made  by  means  of  good  quality  of  pine  boxes,  putty,  and  panes 
of  glass.  A  square  may  be  taken  from  the  middle  of  two  opposite  sides  of  such  a 
box  and  the  window  inserted  in  such  a  way  as  to  give  good  illumination  of  the 
interior.  Running  water  is  even  more  of  a  necessity  here  than  in  the  laboratory. 
A  few  bell  jars,  wire  gauze  cages  for  insects,  boxes  of  various  kinds  for  other 
animals  complete  the  list  of  the  most  essential  features  of  a  good  working  vivarium. 
It  is  always  desirable  to  have  some  green  water-plants  in  the  vessels  of  water  con- 
taining aquatic  animals,  e.g.,  bladder- wort,  watercress,  duck  weed,  and  spirogyra. 

Each  student  should  have  access  to  a  good  compound  microscope.  It  is 
possible  for  two  students  to  work  together  with  one  instrument,  but  such  a  plan 
is  never  very  satisfactory.  At  the  outset  the  teacher  should  give  careful  instruc- 
tions to  the  student  in  the  use  and  care  of  the  compound  microscope.  The 
laboratory  should  also  supply  dissecting  pans  of  heavy  tin,  six  or  eight  by  twelve 
inches,  with  flaring  sides,  and  one  and  one-half  inches  deep.  Pour  into  these  a 
small  amount  of  melted  paraffin  mixed  with  lampblack.  This  forms  an  excellent 


526  ZOOLOGY 

bottom  for  pinning  specimens  to  be  dissected.  There  should  also  be  a  bone 
cutter,  a  syringe  with  rubber  tubing  and  glass  canulas  for  injecting  the  blood  vessels, 
a  supply  of  small  pipettes,  a  few  pipettes  with  large  bulb,  and  two  or  three  flat- 
bottomed  watch  glasses  for  each  student. 

Each  pupil  should  have,  in  addition,  a  good  hand  lens;  a  scalpel;  a  pair  of  fine- 
pointed  scissors;  a  pair  of  forceps;  a  probe;  dissecting  needles;  a  small  supply  of 
glass  slides  and  cover-glasses. 

(d)  Reagents. — The  number  of  necessary  reagents  for  a  beginner's  course  is  not 
large.  The  following  are  the  most  essential. 

Preserving  Reagents.  Alcohol. — This  is  the  most  used  of  all  reagents.  It  is  a 
preserving  fluid.  It  hardens  organic  matter  by  withdrawing  the  water  from  it. 
Commercial  alcohol  is  usually  of  a  strength  of  about  90  to  95  per  cent.  Specimens 
should  first  be  placed  in  50  per  cent,  alcohol  and  then  in  a  day  or  two  be  trans- 
ferred to  a  stronger  grade  (70  per  cent.).  After  such  treatment  they  may  be 
preserved  permanently  in  a  strength  of  70  to  80  per  cent.  Plenty  of  the  preserva- 
tive must  be  supplied,  and  care  must  be  taken  that  it  does  not  lose  too  much 
strength  by  evaporation.  Animals  must  be  opened,  so  that  the  fluid  may  the  more 
quickly  enter  the  cavities. 

Alcohol  may  be  secured  free  of  the  revenue  tax  by  incorporated  institutions,  by 
application  to  the  collector  of  internal  revenue  of  the  district  in  which  the  school 
is  located.  Application  should  be  made  several  months  before  the  alcohol  is 
needed. 

Formaldehyde  has  been  much  used  in  recent  years  as  a  substitute  for  alcohol,  or 
in  combination  with  it,  as  a  preservative.  It  may  be  obtained  as  a  40  per  cent, 
solution,  and  be  further  reduced  by  adding  from  10  to  20  times  its  volume  of  water. 
This  gives  in  the  neighborhood  of  4  per  cent,  to  2  per  cent,  solution  and  the  result- 
ing fluid  will  safely  preserve  materials  through  the  term.  The  same  care  must 
be  observed  as  with  alcohol.  The  specimens  should  be  washed  in  water  before 
studying,  asformol  is  irritating  to  the  mucous  membranes  of  nose,  throat,  and  eyes. 

Killing  Reagents. — Choloroform  is  usually  used  as  a  stupefying  reagent.  Air- 
breathing  animals  exposed  to  its  fumes  are  soon  rendered  unconscious,  and  die  in 
a  relaxed  condition. 

Minute  water  animals  as  Hydra,  Dero,  and  the  like,  are  often  advantageously 
killed  by  sudden  immersion  in  hot  water  or  hot  corrosive  sublimate  (saturated  solu- 
tion). 

Staining  Reagants. — A  few  stains  are  of  advantage,  if  there  is  any  attempt  to 
study  tissues  or  the  Protozoa. 

Magenta  (aqueous  solution).  One  part  by  weight  of  the  dry  magenta  or  fuchsin 
in  100  parts  of  water.  Stains  fresh  tissues  well,  but  is  not  a  nuclear  stain. 

Methyl  green;  one  per  cent,  aqueous  solution.  Add  one  part  of  acetic  acid  to  100 
parts  of  this.  The  resulting  fluid  is  a  superior  nuclear  stain  for  elementary 
work. 

Mounting  Reagents. — Water,  alcohol  of  different  strengths,  glycerine,  and 
normal  salt  solution  (  %  per  cent,  solution  of  common  salt)  are  the  more  commonly 
used  materials  for  temporary  mounting  of  objects  to  be  examined  under  the 
microscope.  The  normal  salt  solution  is  especially  valuable  for  delicate  fresh 
tissues,  blood,  and  the  like. 

The  teacher,  if  inexperienced  in  technic,  must  consult  works  on  microscopical 
methods  for  information  about  the  making  of  permanent  mounts. 


LABORATORY   SUGGESTIONS  527 

Beside  the  materials  mentioned  above  it  is  often  desirable  to  have  other  sub- 
stances,— as  sugar,  acids,  salts,  and  some  of  the  oils,  asxyolol,  benzol,  and  the  like. 
These  should  be  added  gradually  as  their  necessity  and  uses  become  apparent. 

Injection  Masses. — For  the  study  of  the  veins  and  arteries  and  other  tubular 
structures  it  is  often  desirable  to  inject  into  them  foreign  substances  which  prevent 
their  collapse  and  render  them  easy  of  identification.  For  this  purpose  a  syringe 
and  some  rubber  tubing  and  small  canulas  are  necessary.  Injection  masses  to  be 
satisfactory  should  be  fluid  when  injected  and  be  able  to  "set"  or  harden,  after 
injection.  For  ordinary  work  the  following  will  serve: 

1.  Starch  injection  mass: 

Dry  laundry  starch I  volume. 

2M  Per  cent,  aqueous  solution  chloral  hydrate I  volume. 

95  per  cent,  alcohol Y^  volume. 

Coloring  mixture K  volume. 

The  coloring  mixture  is  prepared  by  mixing  equal  parts  of  95  per  cent,  alcohol, 
glycerine,  and  dry  carmine  (vermilion,  chrome  yellow  or  Prussian  blue).  The 
solid  color  should  be  ground  into  small  portions  of  the  fluids  in  a  mortar  so  that  no 
lumps  will  be  present  in  the  mass.  This  mixture  does  not  spoil  with  age,  but  must 
always  be  well  stirred  before  using  and  the  injecting  must  be  rapidly  done,  as  the 
solids  settle  quickly. 

2.  Gum  or  Gelatine  Injection  Masses. — It  is  often  desirable  to  have  a  mass  which 
can  be  forced  through  the  finer  vessels,  as  the  blood  capillaries,  so  that  the  arteries 
and  veins  may  both  be  filled  by  one  injection  into  an  artery  near  the  heart.     The 
following  solution  if  injected  warm  will  pass  the  capillaries.     If  the  gelatine  solution 
is  first  injected  and  then  followed  by  a  starch  mass  of  a  different  color,  the  veins  will 
ultimately  contain  the  former  and  the  arteries  the  latter,  as  the  starch  will,  not 
pass  the  capillaries,  and  thus  both  may  be  easily  studied  because  of  the  contrast 
in  color. 

Gelatine  solution  (i  part  gelatine  to  6  or  8  of  water) I  volume. 

Glycerine  carmine. %  volume. 

Chloral  hydrate  (concentrated  solution) 

2  per  cent.,  by  weight,  of  the  entire  mass. 

The  gelatine  should  be  soaked  in  cold  water  and  then  slightly  heated  until 
dissolved.  The  glycerine  carmine  may  be  prepared  as  follows:  thoroughly  pul- 
verize and  mix  3  grams  of  carmine  with  a  little  water,  with  enough  ammonia  added 
to  dissolve  the  carmine.  Add  50  grams  of  glycerine.  Mix  and  filter.  Add 
gradually  to  this  mixture  enough  acidulated  glycerine  (glycerine  and  acetic  acid 
in  the  ratio  of  10  to  I)  to  give  a  slight  acid  reaction  to  the  carmine  glycerine  mass. 

5.  Materials  for  Study. — The  types  of  animals  needed  for  this  course,  with  the 
exception  of  the  marine  representatives,  may  be  secured  in  almost  any  locality, 
if  sought  at  the  proper  time.  The  teacher  should  become  entirely  familiar  with  the 
common  animals  to  be  found  within  a  reasonable  distance  from  his  school.  It 
is  especially  necessary  to  know  the  life  most  abundant  in  the  various  ponds,  lakes 
and  streams.  A  close  watch  should  be  kept  on  the  material  gathered  from  each 
place,  and  a  record  kept  of  the  various  localities  in  which  each  useful  type  has 


528  ZOOLOGY 

been  found  and  of  the  best  time  for  collection.  In  time  the  laboratory  will  come 
to  have  an  interesting  set  of  facts,  valuable  not  merely  in  assisting  in  the  finding  of 
needed  material,  but  as  indicating  local  distribution  (see  also  §  522;  IV,  4).  The 
students  should  be  encouraged  to  make  excursions,  both  with  and  without  the 
teacher,  to  collect  material  and  extend  the  knowledge  of  the  locality. 

If  for  any  reason  living  materials  cannot  be  secured  in  the  locality  of  the 
school,  preserved  specimens  of  marine,  fresh-water,  and  terrestrial  species  may  be 
secured  of  dealers.  The  principal  are: 

Supply  department,  Marine  Biological  Laboratory,  Wood's  Holl,  Mass.  (Pre- 
served materials.) 

Henry  M.  Stephens,  Carlisle,  Penna.     (Preserved  and  living  material.) 

C.  S.  Brimley,  Raleigh,  N.  C.  (Preserved  and  living:  frogs,  turtles,  alligators, 
etc.,  in  the  winter.) 

The  Anglers'  Bait  and  Manufacturing  Co.,  333-335  W.  South  Water  St., 
Chicago,  111. 

Mr.  A.  A.  Sphung,  North  Judson,  Indiana.  Frogs,  turtles,  clams,  and  crayfish 
(living). 

Dr.  F.  D.  Lambert,  Tufts  College,  Mass.  (South  Harpswell,  Maine,  from 
June  12  to  September  15).  (Preserved  marine  material.) 

Mr.  George  K.  Cherrie,  Brooklyn  Institute  Museum,  Brooklyn,  N.  Y.  (Pre- 
served material.) 

Ward's  Natural  Science  Establishment,  Rochester,  N.  Y. 

Biological  Supply  Co.,  Rochester,  N.  Y. 

Most  of  these  dealers  issue  price  lists  which  may  be  had  on  application. 

In  addition  to  such  materials  as  indicated  above,  unless  the  instructor  has  the 
time  and  equipment  to  make  satisfactory  permanent  mounts  of  microscopic 
preparations,  he  should  secure  a  few,  illustrative  of  cell  structures,  cell  division, 
cleavage  of  ova;  also  sections  of  hydra,  of  the  earthworm,  and  preparations  of 
some  of  the  more  important  tissues  of  higher  animals,  as  bone,  nerve  cells  and  fibres, 
epithelial  tissue,  glandular  tissue  and  the  like.  Some  of  these  may  be  purchased 
of  the  dealers  in  microscopical  supplies.  They  may  usually  be  secured  at  reason- 
able rates  by  writing  to  the  biological  departments  of  the  large  universities. 
There  are  usually  advanced  students  in  these  laboratories  who  are  glad  to  make  a 
few  dollars  in  connection  with  their  work.  Such  preparations  lend  a  great  deal 
of  interest  as  demonstrations  in  connection  with  the  laboratory  work. 

The  writer's  laboratory  (Millikin  University,  Decatur,  111.),  will  be  able  to 
furnish  to  teachers  a  limited  number  of  sets  of  the  microscopic  slides  called  for  in 
this  book. 

Saint  Louis  Biological  Laboratory,  St.  Louis,  Mo.  (Microscopic  and  lantern 
slides.) 

Powers  and  Powers,  Station  A,  Lincoln,  Nebr.  (Wonderfully  fine  microscopic 
slides  of  certain  invertebrate  types.) 

6.  Laboratory  Records. — For  making  these  the  student  should  have  a  note- 
book of  unruled  drawing  paper  of  good  quality,  which  may  be  had  in  a  tablet  or 
keptas  separate  sheets  in  an  appropriate  envelope;  and  good  drawing  pencils,  kept 
sharp,  and  of  hardness  suited  to  the  paper.  In  the  note-book  the  student  should 
keep,  concisely  and  in  an  orderly  way,  the  record  of  all  his  observations,  experi- 
ments, comparisons  and  conclusions.  The  notes  may  be  kept  on  detached  sheets 
similar  to  those  used  for  the  drawings,  if  desired. 


LABORATORY   SUGGESTIONS  529 

Outline  drawings  and  diagrams  must  be  made  of  every  structure  or  relation 
which  can  be  shown  by  a  well-labeled  sketch.  Shading  should  be  sparingly  used 
and  only  with  a  matured  purpose,  the  result  first  being  tested  on  a  separate 
sheet  of  paper.  The  name  of  each  portion  of  the  sketch  should  be  determined  and 
named  by  running  a  leader  from  the  part  to  an  appropriate  place  for  the  name. 
The  drawings  are  always  to  be  made  in  the  laboratory  and  from  the  specimen  studied. 
It  is  through  the  judicious  criticism  of  the  drawings  that  the  teacher  can  best 
bring  out  the  deficiencies  in  the  student's  observations.  One  teacher  cannot  do 
justice  to  a  laboratory  section  of  more  than  twelve  or  fifteen  students.  A  good 
portion  of  the  failure  accredited  in  some  quarters  to  the  laboratory  method  is 
due  to  inefficient  direction.  The  laboratory  will  no  more  run  itself  than  will  the 
class  room. 

The  drawings,  however,  are  by  no  means  the  most  important  part  of  the 
student's  notes.  It  is  more  difficult  and  more  valuable  for  a  student  to  record  his 
observations  and  conclusions  in  good,  concise,  exact  English  sentences  than  in 
drawings. 

It  is  very  desirable  that  students  keep  a  field  note-book,  of  size  suitable  for  the 
pocket,  in  which  all  his  own  outdoor  observations  should  be  entered  and  dated. 
These  notes  may  be  put  into  fuller  form  in  the  reports  called  for  in  the  body  of  the 
text.  It  is  chiefly  through  the  encouragement  of  such  work  as  this  that  the  teacher 
may  hope  to  develop  in  his  students  a  permanent  interest  in  natural  history, 
which  will  contribute  materially  to  their  satisfaction  in  later  life.  It  is  thus  that 
men  and  women  come  to  devote  their  lives  to  nature  study. 

7.  Library  Helps. — The  library  is  quite  as  necessary  to  a  balanced  course  of 
zoology  as  the  textbook,  the  teacher,  or  the  laboratory.  First  under  this  head 
may  be  considered  charts.  The  teacher  should  become  as  expert  as  possible  in 
making  diagrams  on  the  board  before  the  eyes  of  the  pupils.  These  may  be  sup- 
plemented by  charts  made  by  the  teacher,  or  the  pupils,  by  enlarging  figures  found 
in  the  textbooks.  Such  diagrams  have  a  distinct  advantage  over  the  originals 
in  that  they  may  be  discussed  while  in  view  of  the  whole  class.  It  is  excellent 
practice  for  the  pupils  to  make  copies  of  tables  and  figures  for  class  use.  For 
this  purpose  a  good  quality  of  light-colored  wrapping  paper  will  serve,  if  better 
drawing  paper  cannot  be  had.  Keuffel  and  Esser  (New  York  and  Chicago)  will 
send  samples  of  drawing  paper  on  application.  The  outlines  should  be  made  in 
water-colors  with  a  suitable  brush,  in  lines  heavy  enough  to  be  clearly  visible 
across  the  room.  Colors  may  be  put  on  with  crayon  and  fixed  by  a  spray  of 
shellac. 

Photographs  and  lantern  slides  are  of  value  in  illustrating  the  structure, 
development,  and  habits  of  animals.  If  the  school  can  command  a  lantern  or  a 
heliopticon,  a  collection  of  lantern  slides,  selected  in  accordance  with  the  special 
interests  of  the  teacher  and  pupils,  becomes  a  great  stimulus  in  natural  history  work. 
If  a  large  collection  of  books  is  impossible  the  brief  lists  below  will  assist  the  teacher 
in  selecting  the  most  helpful  reference  books  for  an  elementary  course.  More 
extended  bibliographical  lists  will  be  found  in  many  of  the  books  cited.  A  very 
good  working  collection  of  books  may  be  secured  for  about  $150  to  $200. 

In  every  written  report  demanding  library  work  it  is  desirable  to  have  the 
student  record  in  his  paper  a  list  of  all  the  references  bearing  on  the  subject.  It 
is  customary  to  arrange  the  authorities  alphabetically,  together  with  such  other 
facts  as  are  needed  for  ready  reference.  The  following  illustration  will  serve  to 
ndicate  what  facts  should  be  recorded :  ^ 

34 


530  ZOOLOGY 

Parker  and  Has  well. 

'97.     A  Textbook  of  Zoology.     Vol.  I,  pp.  580-583,  illustrations. 

In  addition  to  the  books  listed  below  the  teacher  should  endeavor  to  secure, 
through  his  representative  in  congress,  the  publications  of  the  U.  S.  Department 
of  Agriculture:  the  Yearbook,  the  Farmers'  Bulletins,  the  bulletins  of  the  Bureau 
of  Animal  Industry  and  of  the  Division  of  Entomology.  The  Reports  and  Bulletin 
of  the  U.  S.  Fish  Commission  contain  much  valuable  material.  The  publications 
of  the  state  surveys  and  of  the  experiment  stations  of  the  various  states  are  often 
of  very  high  value  to  the  teacher  and  are  usually  distributed  gratuitously. 

It  will  be  helpful  to  make  as  large  a  collection  as  possible  of  the  current  ele- 
mentary texts  of  zoology,  and  adopt  the  best  suggestions  of  each. 

BIBLIOGRAPHY 

I.  GENERAL:   ANATOMY,    EMBRYOLOGY,    PHYSIOLOGY,    ECOLOGY,    HABITS,    ETC. 

1.  Thompson,  J.  A.     Outlines  of  Zoology.     Fifth  edition.     1913.     D.  Appleton 

and  Co.,  New  York.     Price  $4.50. 

2.  Parker  and  Haswell.     Textbook  of  Zoology.      2   vols.     1897.     The  Mac- 

millan  Co.,  New  York.     Price  $9. 

3.  Hertwig,  R.     General  Principles  of  Zoology.     Translated   by   Field,    1896. 

Henry  Holt  and  Co.,  New  York.     Price  $1.60. 

4.  Hegner,  R.  W.     College  Zoology.     1912.     The  Macmillan  Co.,  N.  Y. 

5.  Jordan,    D.    S.     Animal   Life.     1900.     D.    Appleton    and    Co.,    New   York. 

Price  $1.20. 

6.  Verworn,  M.     General  Physiology;  An  Outline  of  the  Science  of  Life.     Trans- 

lation, 1899.     The  Macmillan  Co.,  New  York.     Price  $4. 

7.  Abbott,  J.  F.     General  Biology.     1914.     The  Macmillan  Co.,  N.  Y. 

8.  Wilson,  E.  B.     The  Cell  in  Development  and  Inheritance.     Second  Edition. 

1900.     The  Macmillan  Co.,  New  York.     Price  $3. 

9.  Wallace,    A.    R.     Geographical    Distribution    of    Animals.     2    vols.     1879. 

Harpers,  New  York.     Price  $10. 

10.  Haddon,    A.    C.     Introduction    to    the    Study    of    Embryology.     1887.     P. 
Blakiston's  Son  and  Co.,  Philadelphia. 

II.  Morgan,   T.   H.     The   Development  of  the   Frog.     1897.     The   Macmillan 

Co.,  New  York.     Price  $1.60. 

12.  Bulletin  U.  S.  National  Museum,  No.  39  (parts  A  to  M  thus  far  published). 

Directions  for  collecting  and  preserving  various  kinds  of  animals.     Wash- 
ington, D.  C.     Price,  a  few  cents  for  each  part. 

13.  Marshall   and   Hurst.     Practical   Zoology.     1888.     G.    P.    Putman's   Sons, 

New  York;  Smith,  Elder  and  Co.,  London.     Price  $3.50. 

14.  Lubbock,  J.     On  the  Senses,  Instincts,  and  Intelligence  of  Animals.     (Int. 

Sci.  Ser.)     D.  Appleton  and  Co.,  New  York,  1888.     Price  $1.75. 

15.  Hornaday,    W.    T.     Taxidermy    and    Zoological    Collecting.     1897.     Chas. 

Scribner's  Sons,  New  York.     Price  $2.50. 

16.  Hegner,  Robert  W.     Practical  Zoology.     1915.     The  Macmillan  Co.,  N.Y. 

17.  Morgan,  C.  L.     Habit  and  Instinct.     1896.     Edward  Arnold,   New  York. 

Price  $4. 

1 8.  Poulton,  E.  B.     The  Colors  of  Animals.     (Int.  Sci.  Ser.)     1890.     D.  Appleton 

and  Co.,  New  York.     Price  $1.75. 

19.  Wallace,  A.  R.     Tropical  Nature.     1895.     The  Macmillan  Co.     Price  $1.75. 


LABORATORY   SUGGESTIONS    •  531 

20.  Darwin,  Chas.     The  Variation  of  Animals  and  Plants  under  Domestication. 

2  vols.     1875.     D.  Appleton  and  Co.,  New  York.     Price  $5. 

21.  Le  Conte,  J.     Outline  of  the  Comparative  Physiology  and  Morphology  of 

Animals.     1900.     D.  Appleton  and  Co.,  New  York.     Price  $1.80. 

22.  Clark,  C.  H.     Practical  Methods  in  Microscopy.     1896.     D.  C.  Heath  and 

Co.,  Boston.     Price  $1.60. 

23.  Beddard,  F.  E.     A  Textbook  of  Zoogeography.      1895.      Cambridge  (Eng.) 

Univ.  Press.     Price  $1.50. 

24.  Calkins,  G.  N.     Biology.     1914.     Henry  Holt  and  Co.,  New  York. 

25.  Eimer,    G.   H.   T.     Organic   Evolution.     1890.     The   Macmillan   Co.,    New 

York.     Price  $4. 

26.  Shttfer,  E.  A.     Essentials  of  Histology.     Fourth  Edition.     1894.     Lea  Bros., 

Philadelphia. 

27.  Shipley,   A.   E.     Zoology  of  the  Invertebrates.     1893.     A.   and   C.   Black, 

London.     Price  $6.25. 

28.  Van  Beneden,  F.     Animal  Parasites  and  Messmates.     1876.     D.  Appleton 

and  Co.,  New  York.     Price  $1.50. 

29.  Schmeil,  Otto.     Textbook  of  Zoology,  from  a  Biological  Standpoint.     1901. 

A.  and  C.  Black,  London.     Price  $4. 

30.  Hodge,  C.  F.     Nature  Study  and  Life.     1902.     Ginn  and  Co.,  Boston. 

•31.  Scott,  Chas.  B.     Nature  Study  and  the  Child.     1902.     D.  C.  Heath  and 
Co.,  Boston. 

32.  Morgan,  T.  H.     Heredity  and  Sex.     1913.     Columbia  University  Press,  N.  Y. 

33.  Davenport,  C.  B.    Heredity  in  Relation  to  Eugenics.    Henry  Holt  and  Co.,  N.Y. 

34.  Locy,  Wm.  A.     Biology  and  its  Makers.     1908.     Henry  Holt  and  Co.,  N.  Y. 

35.  Hegner,  R.  W.     The  Germ-cell  Cycle  in  Animals.     1914.     The  Macmillan 

Co.,  N.  Y. 

36.  Riverside    Natural   History:   J.    S.    Kingsley,    Editor.     6   vols.     Houghton, 

Mifflin  and  Co.,  Boston.     Price  $30. 

II.  SPECIAL:  BOOKS  TREATING  THE  STRUCTURE  AND  CLASSIFICATION  OF  THE 
MORE  IMPORTANT  GROUPS  OF  ANIMALS 

1.  Jordan,  D.  S.     Manual  of  the  Vertebrates  of  the  Northern  United  States. 

Eighth  Edition.     1899.     A.  C.  McClurg  and  Co.,  Chicago.     Price  $2.50. 

2.  Chapman,  F.  M.     Bird  Life.     A  Guide  to  the  Study  of  Our  Common  Birds. 

1897.     D.  Appleton  and  Co.,  New  York.     Price  $2. 

3.  Chapman,  F.  M.     Hand-book  of  Birds   of  Eastern  North  America.     1900. 

D.  Appleton  and  Co.,  New  York.     Pricefo. 

4.  Comstock,    J.    H.      Manual    for    the    Study    of    Insects.     1895.     Comstock 

Publishing  Oo.,  Ithaca,  New  York.     Price  $3.75. 

5.  Folsom,  J.  W.     Entomology:  with  special  reference  to  its  Biological  and 

Economic    Aspects.     1906.     P.   Blakiston's    Son    and   Co.,   Philadelphia. 
Price  $3. 

6.  Lubbock,  J.     Ants,  Bees  and  Wasps.     Int.  Sci.  Series.     1882.     D.  Appleton 

and  Co.,  New  York.     Price  $2. 

7.  Emerton,  J.  H.     Common  Spiders.     1902.     Ginn  and  Co.,  Boston,  Mass. 

Price  $1.50. 

8.  Herrick,  F.  H.     The  American  Lobster:  Its  habits  and  development.     1896. 

Bulletin  United  States  Fish  Commission,  Vol.  XV. 


532  ZOOLOGY 

9.  Darwin,  Chas.     Vegetable  Mould  and  Earthworms.     D.  Appleton  and  Co., 
New  York.     Price  $2. 

10.  Calkins,  Gary  N.     The  Protozoa.     1901.     The  Macmillan  Co.,  New  York. 

Price  $3. 

11.  Howard,    L.    O.     Mosquitos.     1901.     McClure,    Phillips     and     Co.,    New 

York.     Price  $1.50. 

12.  Baskett,   J.   N.     The  Story  of   the  Birds.     1899.     D.   Appleton  and   Co., 

New  York.     Price  65  cents. 

13.  Cowan,    T.    W.     Natural    History    of    the    Honey    Bee.     1890.     Houston, 

London,     is.     6d. 

14.  Holland,  W.  J.     The  Butterfly  Book.     1899.     Doubkday  and  McClure  Co., 

New  York.     Price  $3. 

15.  Holland,   W.   J.     The  Moth  Book.     1904.     Doubleday  and   McClure  Co., 

New  York.     Price  $3. 

16.  Comstock,  J.  H.     Insect  Life.     1901.     D.  Appleton  and  Co.     Price  $1.50. 

17.  Sanderson,   E.   D.    Insect    Pests    of   Farm,    Orchard,  and  Garden.     1912. 

John  Wiley  and  Son,  N.  Y. 

18.  Howard,  L.  O.    The  House  Fly,  Disease  Carrier.    1911.    Stokes,  New  York. 

Price  $1.60. 

8.  Collections. — While  the  educative  value  of  a  miscellaneous  assortment  of 
the  curios  so  often  brought  to  teachers  is  not  great,  a  permanent  collection  of  the 
typical  animals  of  the  locality  may  be  so  arranged  as  to  be  of  considerable  value 
for  comparison.  There  should  be  added  to  these  gradually,  by  purchase  or  other- 
wise, representatives  of  those  classes  of  animals  not  represented  in  the  local  fauna 
in  order  to  give  the  collection  more  of  a  synoptic  value.  Such  a  collection  of 
types  from  the  more  important  classes  of  animals  serves  an  important  end  in  giving 
the  student  a  general  idea  of  the  animal  kingdom  as  a  whole,  which  is  difficult  to 
gain  in  any  other  way.  The  building  up  of  a  laboratory  museum  with  the  help  of 
the  students  may  be  made  to  serve  as  an  incentive  to  care  and  neatness  on  their 
part  in  making  dissections  or  other  preparations  which  may  bear  the  name  of  the 
student  on  the  labels,  when  permanently  installed  in  the  collection. 

It  is  decidedly  valuable  to  encourage  students  in  the  special  study  of  some 
limited  group  of  animals,  and  this  may  frequently  be  accomplished  by  the  begin- 
nings of  a  collection  of  the  local  species  of  the  group.  The  permanent  interest 
and  enthusiasm  on  the  part  of  the  pupil  in  the  study  of  the  phenomena  of  living 
things  may  be  taken  as  the  measure  of  success  in  teaching  the  natural  history 
sciences.  This  can  be  secured  more  readily  by  studying  life  in  its  natural  sur- 
roundings than  by  dissections. 


INDEX 


[The  names  of  the  genera  cited  are  printed  in  italics.  The  numerals  refer 
to  pages.  Those  in  bold-faced  type  indicate  that  the  object  is  illustrated  on 
that  page.] 


Absorption,  66 

Abyssal  fauna,  135 

Acalephs,  245 

Acanthocephala,  203 

Acanthopteri,  372 

Acclimatization,  13,  105 

Achromatin,  21,  27 

Acquired  characters,  100,  102 

Actinozoa,  180 

Adaptation,  means  of,  105;  types  of, 
in;  to  inorganic  environment,  112; 
to  mates,  114;  to  offspring,  115; 
social,  120;  competitive,  121;  com- 
mensal, 122;  symbiotic,  122;  to  prey- 
ing, 125;  for  protection,  125 

Adaptive  habits  and  instincts,  132 

Adipose  tissue,  55 

Adrenals,  341 

Aepyornis,  412 

Aerial  fauna,  136 

Age  of  man,  472 

Air-sacs  (birds),  404 

Aix,  412 

Albatross,  414 

Alimentary  canal  (see  digestive  sys- 
tem). 

Allantois,  391,  408 

Alligator,  396 

Allolobophora,  222 

Alternation  of  generation — in  Coelen- 
terates,  178;  in  worms,  191,  196 

Ambulacral  system,  213 

Amitotic  cell  division,  24 

Ammonites,  262 

Amnion,  391,  408 

Amceba,  15,  23,  92,  149 

Amphibians,  144,  375 

Amphioxus,  312,  313 


Amphitrite,  236 

Amphiuma,  380 

Ampullae,  214 

Anabolism,  66 

Anacanthini,  373 

Analogy,  84 

Anaphase,  25 

Anatomy,  3,  511 

Animalcules  (see  protozoa). 

Animal  industries,  505 

Annelids  (see  annulata). 

Annulata,  146,  222,  236 

Anodonta,  238,  258 

Anseres,  415 

Ant-eater,  451 

Antenna,  267 

Antennule,  269 

Antero-posterior  (see  symmetry) . 

Antimeres,  96 

Ants,  121,  306 

Anura,  313 

Aorta,  338  (see  circulatory  system). 

Apes,  459 

Aphis  (plant  lice),  299 

Appendages — arrangement   of,    96;     in 

arthropods,  278;  in  vertebrates,  324; 

in  fishes,  359;  in  amphibians,  376; 

in  birds,  401 

Appendicular  skeleton,  330,  332 
Appendix,  445 
Applications  of  zoology,  510 
Aptera,  297 
Apteryx,  406,  410 
Aquatic  fauna,  135 
Arachnida,  306 
Araneida,  307 
Arbacia,  209 
Archceopteryx,  409 


533 


534 


INDEX 


Archenteron,  39-40 

Archipterygium,  360 

Ardea,  414 

Argonauta,  263 

Aristotle's  lantern,  210 

Armadillo,  451 

Army- worm,  404 

Arterial  arches,  338 

Ateries,  7i_(see  circulatory  system). 

Arthropoda,  145,  265 

Artificial  selection,  104 

Artiodactyla,  453 

Ascaris,  199  o 

Ascidians,  312 

Asellus,  269 

Ass,  454 

Assimilation,  13  (see  metabolism). 

Asterias,  207 

Asteroidea,  220 

Astroid-stage,  24  (cell- division). 

Asymmetry,  92,  371 

Atrial  pore,  313 

Atrium,  313 

Auditory  nerve,  352 

Auditory  organs,  81,  281,  354 

Auk,  413 

Aurelia,  185 

Autolytus,  229,  230 

Automatism,  12 

Aves,  398,  410 

Axiality,  91,  93 

Axial  skeleton,  328 

Axon,  6 1 

Baboons,  459 
Balanoglossus,  311,  312 
Barbs  (see  feather). 
Barbules  (see  feather). 
Barnacles,  292 
Bass  (sea),  371 
Bats,  457 
Bears,  454 
Beaver,  457 
Bees,  121,  306 
Beetles,  304 

Behavior   (see  sense  organs,   and  sen- 
sation) . 

Bibliography,  530 
Bilateral  symmetry,  95,  97 


Binomial  nomenclature,  510 

Biogenetic  law,  484 

Biographical  zoology,  508 

Bird  lice,  299 

Birds,  142 

Bittern,  415 

Bivalent  chromosomes,  33,  34,  36 

Bivium,  212 

Blastoidea,  219 

Blastomeres,  36 

Blastopore,  38,  39 

Blastula,  38,  39 

Blood,  57 

Boat-shell,  260 

Body-cavity  (see  ccelom). 

Bojanus,  organ  of  (=  kidney  of  clam). 

Bonasa,  417 

Bone,  56 

Bony-fishes,  372 

Book-lungs,  281 

Bot-fly,  300 

Bothriocephalus,  204,  206 

Brachiopoda,  203 

Brain:  orang,  467;  Hottentot,  468; 
Gauss,  468 

Brain  (vertebrate),  347,  348 

Branchiae,  69 

Branchial  arches,  339-341  (see  circula- 
tory system). 

Bronchi,  446 

Brook  trout,  368 

Bryozoa  (see  polyzoa,  202). 

Buccal  cavity,  333 

Bugs,  299 

Bubo,  420,  421 

Bufo,  381 

Butterflies,  302 

Cabbage  butterfly,  303 
Cabbage  worm,  303 
Caecum,  445 
Cambarus,  265 
Camel,  453 
Camera  eye,  82,  357 
Campanularia,  185 
Camp  odea,  298 
Canine  teeth,  443 
Canker-worm,  304 
Capillaries,  71,  339 


INDEX 


535 


Caprella,  296 

Carapace — in   Crustacea,   282;   turtles, 
392 

Carbohydrates,  1 1 

Cardinal    veins,    364    (see    circulatory 
system). 

Cardium,  258 

Carinatae,  412 

Carnivora,  450,  454 

Carpels,  332 

Cartilage,  54,  55 

Cat,  454 

Caudal  vertebrae,  330 

Cecidiomya,  302 

Cell,  19 

form,  19;  size,  20;  structure,  20; 
wall,  21 ;  functions,  22;  reproduc- 
tion, 23;  division,  23 

Centipede,  296 

Central  nervous  system,  77,  347 

Centropiistis,  371 

Centrosome  (centrosphere),  26 

Centrum  (see  vertebra),  328 

Cephalopoda,  262 

Cephalo-thorax,  278 

Ceratodus,  374 

Cercaria,  196 

Cerebellum,  348,  350 

Cerebrum,  348,  350 

Cervical  vertebras,  329 

Ceryle,  423 

Cestodes,  198 

Cetacea,  452 

Chaetognatha,  203,  238 

Chaetopoda,  236 

Chameleon,  393 

Cheiroptera,  450,  457 

Chelonia,  392 

Chemical  sense,  80,  353  (see  taste  and 
smell). 

Chicken,  416;  breeds  of,  434 

Chimney-swifts,  429 

Chlamydomyxa,  22 

Chordata,  311   (see  vertebrata). 

Chorion,  408,  448 

Chromatin,  21,  27 

Chromosomes,  24 

Cicada,  299 

Cilia,  22,  43,  147 


Circulation,  66,  70 

Circulatory  organs,  70;  in  Echino- 
derms,  215;  in  annulata,  230;  in 
mollusks,  250;  in  arthropods,  283; 
in  vertebrates,  337,  343;  in  fishes, 
364,  365;  in  amphibia,  340,  378;  in 
reptiles,  340,  388;  in  birds,  341;  in 
mammals,  341,  445 

Cirratulus,  237 

Cirripedia,  293      9 

Cistudo,  392 

Clam,  239,  256 

Clamatores,  429 

Classification,  485 

Classification — defined,  5;  use,  107; 
illustration  of,  no 

Classification — protozoa,  161;  porifera, 
171;  ccelenterata,  185;  unsegmented 
worms,  195;  annulata,  236;  mollusca, 
257;  arthropoda,  292;  vertebrata, 
357;  fishes,  372;  amphibia,  383; 
reptiles  392;  birds,  410;  mammals, 

449 

Cleavage,  36,  37,  38 

Clitellum,  223 

Cloaca,  167,  336,  406 

Clothes-moth,  304 

Clothing  from  animals,  499 

Clypeus,  272 

Coccyzus,  424 

Cockroaches,  297 

Cocoon,  288 

Coelenterata,  146,  173 

Coelom  (defined),  41;  in  unsegmented 
worms,  191;  echinoderms,  213;  an- 
nulata, 229;  arthropods,  277;  verte- 
brates, 333 

Coelomata  and  ccelenterata,  146 

Coil  stage  (cell-division),  24,  25 

Coleoptera,  304 

Colonies — organic,  119;  voluntary,  120 

Color,  125,  370 

Colpoda,  20 

Columbae,  423 

Commensalism,  122 

Compound  eye,  83,  285 

Condyle,  387,  400 

Conjugation,  74,  15 

Congo-snake,  383,  386 


536 


INDEX 


Connective  tissue,  53-58 

Contractility,  15  (see  motion). 

Contributors  to  zoology,  509 

Convolutions  (brain),  446,  468 

Coordination,  76 

Corallines,  202 

Corallum,  182 

Corals,  185 

Corpuscles  (blood),  57 

Corrodentia,  298 

Cortex  (of  brain),  446 

Corvidae,  430 

Cotton-boll  worm,  304 

Coverts  (see  feather). 

Coxa,  272 

Crab,  289 ;  larvae,  287,  288 

Crane,  417 

Cranial  nerves,  352 

Crayfish,  265,  293 

Crepidula,  260 

Crickets,  297 

Crinoidea,  219 

Crocodiles,  396 

Crop,  334 

Crotalus,  387 

Crows,  425 

Crustacea,  292 

Ctenophora,  185 

Cuckoo,  424 

Cursores,  411 

Cyclas,  258 

Cyclops,  269,  292 

Cyclostomes,  313 

Cyrtophyllus,  299 

Cyst,  158 

Cystoidea,  219 

Cytology,  512 

Cytoplasm,  20,  27 


Damsel  flies,  298 
Daphnia,  292 
Decapoda,  263 
Deer,  453 
Degeneration,  130 
Dendron,  61 
Dental  formula,  443 
Dentine,  50,  443  (teeth). 
Dermaptera,  299 


Dermis,  326  (see  integument). 

Dermo-muscular  sac,  228 

Dero,  224,  226 

Derotremata,  383 

Development  (see  metamorphosis) — 
porifera,  170;  ccelenterata,  183;  un- 
segmented  worms,  191,  196;  echino- 
derms,  218;  annulata,  233;  mollusks, 
253;  arthropods,  286;  vertebrates, 
345i  346;  fishes,  369;  amphibians, 
379;  reptiles,  390;  birds,  407;  mam- 
mals, 447 

Development  of  zoology,  508 

Devil-fish,  262 

Diastroid  stage,  24,  25 

Dibranchiata,  262 

Didelphia,  450 

Didelphys,  115 

Differentiation  of  tissues  and  organs, 
29,  41,  47,  84 

Digestion,  66,  67 

Digestive  system — protozoa,  154; 
sponges,  169;  ccelenterata,  180;  echi- 
noderms,  212;  annulata, 229 ;  mollusks, 
248;  arthoropoda,  280;  vertebrates, 
33 it  39i;  birds,  404;  mammals,  443; 
ruminants,  444 

Digestive  tract,  regions,  333 

Dimorphism  (sexual),  75 

Dinosaurs,  385;  485 

Dipnoi,  373 

Diptera,  300 

Direct  cell-division,  23 

Discophora,  238  (leeches). 

Dispersal  of  animals,  134 

Dissimilation,  16  (katabolism). 

Distomum,  196 

Distribution,  geographical,  4,  136 

Division  (cell),  23,  25 

Division  of  labor,  29,  65 

Dogs,  454 

Domestication  of  animals,  105,  504 

Dominance,  493 

Dorsal  vertebrae,  329 

Dorso-ventral  axis  (see  symmetry). 

Dove,  423 

Draco,  394 

Dragon  flies,  298 

Duck-bill,  439 


INDEX 


537 


Ducks,  412 
Ductless  glands,  340 
Ductus  Cuvieri,  364 
Dugong,  452 

Eagle,  419 

Ear,  354  (see  auditory  organs) . 

Earthworm,  222 

Earwigs,  299 

Echidna,  449 

Echinodermata,  145,  207 

Echinoidea,  220 

Echinus,  209 

Ecology — defined,  4  (see  also  adapta- 
tion); of  protozoa,  161;  porifera,  171; 
coelenterata,  185;  unsegmented 
worms,  203;  echinoderms,  219;  an- 
nulata,  235;  mollusks,  255;  arthro- 
pods, 289;  fishes,  366,  370;  amphibia, 
379;  reptiles,  391;  birds,  422;  mam- 
mals, 459 

Economic  zoology,  498 

Ectoderm,  38,  39 

Ectosarc,  152 

Edentata,  450,  451 

Education,  470 

Egg,  30 

Elasmobranchii,  372 

Electric  organs,  370 

Elephant,  454 

Elytra,  304 

Embryology,  481 

Embryology,  defined,  4  (see  develop- 
ment), history  of,  513 

Emu,  411 

Enamel,  50  (see  teeth),  442 

Endopodite,  267 

Endosarc,  152 

Endoskeleton,  72,  327,  403  (see  also 
skeleton). 

Endothelium,  52 

Ensis,  257 

Entoderm,  38,  39 

Entomostraca,  292 

Environment,  99,  102,  112,  495 

Epeira,  308 

Ephemerida,  298 

Epiblast  (see  ectoderm). 

Epicranium,  272 


Epidermis  (see  integument),  325 

Epinephelus,  373 

Epithelial  tissue,  48-53 

Equilibrium  sense,  81,  354 

Erethizon,  456 

Ethnography,  474 

Eugenics,  519 

Euglena,  153,  157 

Eutcenia,  395 

Euthyneura,  260 

Eustachian  tube,  354 

Evolution, — meaning,  476;  evidences, 
479;  course  of,  485;  factors  in,  486 

Excretion,  17,  66,  71 

Excretory  systems — ccelenterates,  1827 
unsegmented  worms,  193,  194;, 
echinoderms,  216;  annulata,  231;. 
mollusks,  251;  arthropods,  283;  ver- 
tebrates, 344 

Exopodite,  267 

Exoskeleton,  72,  324  (see  also  integ- 
ument). 

Eyes,  82,  285,  355  J  median,  389 

Eye-spots,  coelenterata,  183;  annulata, 
232 

Facial  nerve,  352 
False  amnion,  409 
Fat  cells,  55 
Fats,  ii 

Faunas,  type  of,  135 
Feathers,  399,  402 
Femur,  332 
Ferments,  10 
Fertilization,  36,  42,  74 
Fibrillas  (muscle),  59 
Fibrous  connective  tissue,  54,  55 
Fin,  360 
Finches,  420 
Fishes,  144,  315,  358 
Fish  Commission,  work  of,  368 
Fission,  191,  194 
Flame-cell,  192,  194 
Flamingo,  415 

Flat- worms  (see  platyhelminthes,  192) 
Fleas,  300 
Flies,  300 

Fcetal  membranes  (see  amnion  and 
allantois). 


538 


INDEX 


Fluctuations,  488 
Food  value  of  animals,  498 
Foraminifera,  161 
Formative  pole,  37 
Fox,  454-457 
Fringillidae,  430 
Frog,  318,  381 
Frontal  section,  91 
Fruit-borers,  304 
Functions  (basal),  65 
Fur  (see  hair). 

Galea,  273 

Gallinae,  421 

Galls,  115,  116 

Callus,  423 

Gamete,  158 

Gammarus,  295 

Ganglion,  61,  347 

Ganoidei,  372 

Gar-pike,  372 

Gasteropoda,  260 

Gastrophilus,  300,  301 

Gastro-vascular  cavity,  177,  180 

Gastrula,  38,  39 

Gavials,  396 

Geese,  415 

Gelatinous  tissue,  54 

Genae,  272 

Genus,  109 

Germinal  layers,  40 

Germinative  epithelium,  344 

General  characters — protozoa,  152; 
porifera,  165;  ccelenterata,  176;  un- 
segmented  worms,  190;  echino- 
derms,  210;  annulata,  275;  mollusks, 
244,  arthropods,  277;  chordata,  311; 
vertebrates,  323;  fishes,  358;  am- 
phibia, 375;  reptiles,  385;. birds,  400; 
mammals,  438 

General  survey — protozoa,  151;  por- 
ifera, 165,  167;  ccelenterata,  178; 
echinoderms,  211;  unsegmented 
worms,  190;  annulata,  226;  mollusks, 
244;  arthropods,  277;  vertebrates, 
cb,  XIX;  fishes,  359;  amphibia,  375; 
reptiles,  385 ;  birds,  401 ;  mammals,  439 

Geographical  distribution,  478 

Gephyrea,  203 


Germ  cell  cycle,  33,  34,  42 

Germ  plasm,  489 

Gila-monster,  393 

Gill-arches,  336 

Gills,  69,  336 

Girdles,  330,  332 

Gizzard,  280,  335,  404 

Glands,  50,  52;  in  mammalia,  441 

Glass-snake,  393 

Glosso-pharyngeal  nerve,  352 

Glottis,  337 

Gnats,  300 

Goblet  cells,  49 

Goldfinch,  426 

Gonads,  183  (see  ovary  and  testis). 

Grallatores,  411 

Grantia,  164 

Grasshopper,  270,  297 

Green  gland,  283 

Grouse,  417,  421 

Growth,  14,  73 

Guinea-fowl,  416 

Gulls,  413,  414 

Gymnophiona,  383 

Habits  and  habitats  (see  ecology),  132, 

459 

Hasmocele,  279 
Haemoglobin,  337 
Hair,  440,  441 
Hair-follicle,  326 
Hares,  457 

Harmful  animals,  503 
Harvest-men,  309 
Hawks,  419 
Haversian  canal,  56 
Hearing,  81  (see  ear). 
Heart,  338  (see  circulatory  system). 
Hedge-hog,  446 
Helix,  246,  261 
Heloderma,  394 
Hemiptera,  299 
Hepatic  portal  circulation,  364 
Heredity,  99,  488;  carriers  of,  100 
Hermaphroditism,  74,  232 
Hermaphrodite     organs — tapeworm, 

198;  snail,  252 
Hermit  crab,  123,  186 
Heron,  414 


INDEX 


539 


Hessian  fly,  301 

Heterocercal  tail,  361 

Heteromya,  259 

Hexapoda,  297 

Hippopotamus,  442 

Historical — general,   6;  protozoa,    159; 

of  zoology,  508 
Histology — defined,  4 
Hog,  453 

Holothuroidea,  220 
Homarus,  293 
Homocercal  tail,  362 
Homology,  84 
Hookworm,  200 
Horned-toad,  393 
Hornbill,  428 
Horse,  453 

Human  evolution,  497 
Humming-bird,  429 
Hybrid,  109 
Hydra,  173,  1 77 
Hydractina,  186 
Hydroid  type,  178 
Hydrozoa,  185 
Hylocichla,  427 
Hymenoptera,  305 
Hypoblast  (see  entoderm). 

Ichneumon  fly,  306 

Icteridae,  430 

Imago,  288 

Incisor  teeth,  442 

Infusoria,  161 

Ingestion  of  food,  66 

Injurious  animals,  503 

Insectivora,  451 

Insects,  297 

Instincts,  132 

Integument  (see  skin) — annulata,  228; 

echinoderms,  212;  mollusks,  242,  246; 

arthropods,    279;    vertebrates,    325; 

fish,  362;  amphibians,  376;  reptiles, 

386;  birds,  401;  mammals,  441 
Intercellular  substance,  48 
Intestine,  335  (see  digestive  system). 
Invagination,  39 

Irritability,  12,  76  (see  sensation). 
Isolation,  495 
Isoptera,  298 


Jackals,  454 

Jelly-fish,  178  (see  medusa). 

Jugular  vein,  364 

Katabolism,  16,  66 

Katydid,  297,  299 

Keeled  birds,  412 

Kidney,  71,  344  (see  nephridia). 

King  crab,  306 

Kingfisher,  423 

Labor-saving  animals,  500 

Laboratory  exercises — protozoa,  149; 
porifera,  164;  ccelenterata,  173;  un- 
segmented  worms,  191;  echinoderms, 
207;  annulata,  222;  mollusks,  239; 
arthropods,  265;  chordata,  315;  fish, 
315;  amphibian,  318;  reptile,  384; 
birds,  398;  mammals,  437 

Labium,  273 

Labrum,  272 

Lacertilia,  392 

Lacinia,  273 

Lacteals,  446 

Lacunae — bone,  56;  blood,  283 

Ladybird  (ladybug)  beetle,  304 

Lamella  (bone),  56 

Lamellibranchiata,  257 

Lamprey,  126,  311 

Lampropeltis,  395 

Lancelet,  312,  313 

Lanius,  429 

Lark,  428 

Larynx,  337 

Leech,  225-238 

Leopard,  454 

Lemurs,  459 

Lepidoptera,  302 

Lepidosiren,  374 

Lepornis,  392 

Leucania,  304 

Lice,  299 

Life  cycle,  42 

Life — nature  of,  8;  relation  to  pro- 
toplasm, 19 

Limax,  261 

Limbs,  bones  of,  332 

Limicolae,  417 

Limnaa  (host  of  liver  fluke,  195),  242 


540 


INDEX 


Limpet,  260 

Limulus,  306 

Lion,  454 

Littoral  fauna,  135 

Littorina,  260 

Liver-fluke,  195,  196 

Lizards,  393 

Lobster,  293,  294 

Locomotion,  75 

Locomotor  organs — protozoa,  154; 
porifera,  169;  coelenterata,  182;  echi- 
noderms,  213;  annulata,  228;  mol- 
lusks,  247;  arthropods,  280;  fishes, 
359-363,  amphibia,  378;  reptiles, 
386;  birds,  401;  mammals,  430 

Locust-borers,  304 

Locusts,  297 

Longipennes,  414  ^ 

Lophobranchii,  375 

Lophophore,  202 

Lumbar  vertebrae,  327 

Lumbricus,  222 

Lung-fishes,  373 

Lungs,  69,  335 

Lymph,  58 

Lymphatics,  445 

Macromere,  233,  254 

Madreporic  body,  214 

Malacostraca,  293 

Malaria,  162 

Mallophaga,  299 

Malpighian  tubule,  281 

Mammals,  142 

Mammary  glands,  441 

Mammoth,  454 

Man,  459,  ch.  XXV 

Man,  465;  types  of,  473 

Manatee,  452 

Mandible,  263,  273 

Mantle,  247 

Marsupials,  440,  439,  450 

Marsupium,  439,  448 

Marten,  454 

Mastodon,  454 

Maturation  (of  ovum),  32,  33,  34 

Maxilla,  267,  273 

Maxillary  palpus,  273 

Maxillipeds,  267 


May-flies,  298 

Medical  uses  of  animals,  501 

Medulla  oblongata,  348,  350 

Medullary  sheath  (nerve),  60,  61 

Medusa-type,  178 

Megalops,  288 

Mendel's  experiments,  489 

Mendel's  laws,  492 

Mental  life  of  man,  469 

Mesenchyma  (mesoglcea),  166 

Mesenteron,  67,  331 

Mesentery,    320 

Mesoblast  (see  mesoderm),  233 

Mesoderm,  40 

Mesothorax,  272 

Mesozoa,  203 

Metabolism,  66 

Metamere,  96 

Metamorphosis — worms,     195,     196; 

echinoderms,   218;  arthropods,  "287; 

amphibia,  379,  380 
Metaphase,  24 
Metapleure,  313 
Metathorax,  272 
Metazoa,  147-485 
Metridium,  175 
Mice,  457 

Micromeres,  233,  254 
Micropyle,  36 
Microstomum,  194 
Migration,  105,  134;  of  birds,  432 
Millipedes,  296 
Mimicry,  128,  390 
Mimus,  427 
Mink,  454 
Mites,  309 

Mitotic  cell-division,  24,  25 
Mniotiltidae,  430 
Mocking  bird,  427 
Modern  birds,  410 
Molar  teeth,  443 
Moles,  451 
Mollusca,  239-145 
Molluscoidea,  202 
Monkeys,  459 
Monodelphia,  450 
Monomya,  259 
Monotremata,  439,  449 
Morphology — defined,  3 


INDEX 


541 


Morula,  38,  39 

Mosquitoes,  301,  302 

Moths,  302 

Motion,  75  (see  locomotion);  proto- 
plasmic, 15 

Moulting,  279 

Mouth-parts  (arthropods),  267,  273 

Mucous  epithelium,  49 

Muscular  system,  75;  coelenterates,  182; 
echinoderms,  216;  annulates,  228; 
mollusks,  247;  arthropods,  280;  verte- 
brates, 345 

Muscular  tissue,  58-60 

Mussel,  252,  254,  239,  258 

Mutation,  107,  488 

Mya,  239,  256 

Myctodera,  383 

Myotome,  313,  345 

Myriapoda,  296 

Mytilus,  258 

Nais,  238 

Natural  history,  510 

Natural  selection,  104 

Nauplius,  293 

Nautilus — pearly  261;  paper,  263 

Necator,  200 

Necturus,  380,  383 

Nemathelminthes,  199 

Nemertinea,  203 

Neornithes,  410 

Nephridia,  71,  72,  231,  344 

Nereis,  225,  236 

Nervous  system,  76-83  (see  sense  or- 
gans); ccelentrata,  182;  echinoderms, 
217;  annulata,  231;  mollusks,  251; 
arthropods,  283;  vertebrates,  347; 
reptiles,  388;  birds,  405;  mammals, 
446 

Nervous  tissue,  60,  61 

Nettling  cells,  181 

Neuron,  61 

Neuroptera,  298 

Newts,  383 

Nictitating  membrane,   355;  in  birds, 

405 

Notochord,  311,  328 
Nucleoplasm,  27 
Nucleolus,  21 


Nucleus,  21 ;  functions  of,  26 
Nutrition,   66;  protozoa,    154;  see  di- 
gestion, metabolism,  etc. 
Nutritive  pole,  37 

Obelia,  185 

Ocelli,  285 

Occulina,  176 

Octopoda,  259 

Octopus,  262,  263 

Odonata,  298 

Odontophore,  244,  248 

(Esophagus,  334 

Oil  glands,  326 

Olfactory  nerves,  352 

Oligochasta,  237 

Ommatidium,  285 

Oniscus,  269 

Onychophora,  296 

Oocytes,  32,  33,  34 

Oogonia,  32,  33,  34 

Ophidia,  393 

Ophiuroidea,  220 

Optic  nerve,  352,  355 

Orb-weavers,  307 

Organization,  19, 47, 65;  of  protozoa,  153 

Organs,  66 

Origin  of  tissues,  47 

Original  home  of  animals,  135 

Orioles,  420 

Ornithodelphia,  449 

Ornithorhynchus,  449 

Orthoceratites,  262 

Orthoptera,  297 

Oscines,  429 

Osculum,  165 

Osmerus,  370 

Osseous  tissue  (bone),  56 

Ostrea,  259 

Ostrich,  411 

Otocyst,  8 1  (see  also  auditory  organs). 

Otolith,  8 1 

Otter,  454 

Ovary,  74,  286,  344 

Oviduct,  252,  286,  344,  406 

Ovipositor,  272 

Oviparous,  345 

Ovum,  30 

Owls,  419,  420,  421 


542 


INDEX- 


OX,  453 

Oxidation,  17,  68 
Oysters,  242,  256,  259 

Palamonetes,  295 

Palaeontology,  479 

Palseozoology,  defined,  5 

Paludicolae,  417 

Papilio,  305 

Parapodia,  236 

Paramecium,  149,  153 

Parasitic  worms,  206 

Parasitic  worms, — cycle  of,  206 

Parasitism,  129,  195,  204,  206 

Parental  care,  115,  460 

Parthenogenesis,  114 

Passeres,  429 

Pea-fowl,  421 

Pearls,  257 

Pearl  oyster,  259 

Pecten,  259 

Pelagic  fauna,  135 

Pelecanus,  416 

Pelecypoda,  257 

Penguin,  413 
Pennaria,  185 

Perennibranchiata,  383 

Pericardial  sinus,  283 

Perichondrium,  55 

Periosteum,  56 

Peripatus,  296 

Peripheral   nervous    system,    79;    347, 

351  (see  nervous  system). 
Perissodactyla,  453 
Peritoneum,  316 
Periwinkles,  260 
Perodipus,  455 
Petrels,  414 
Pharyngognathi,  373 
Pharynx,  333 
Pheasant,  421 
Philosophy  of  biology,  515 
Phasnicopterus,  415 
Pholas,  258 
Phyla,  key  to,  147 
Physalia,  187 

Physiology,  defined,  4;  history  of,  513 
Physopoda,  299 
Physostomi,  372 


Pici,  426 

Pieris,  303 

Pigeon,  417 

Pigments,  n 

Pituitary  body,  341 

Placenta,  448 

Placentalia,  439,  450 

Planarians,  193 

PlanorbiS,  261 

Plant-lice,  299 

Planula,  184 

Plasma  (blood),  57,  338 

Plastron,  392 

Platyhelminth.es,  192 

Plecoptera,  298 

Plectognathi,  373 

Pleura,  337  f 

Pleuron,  266 

Plexus,  352 

Plover,  417,  433 

Plumatella,  192,  202 

Polar  bodies,  33,  34,  35 

Polychaeta,  236 

Polygordius,  234 

Polymorphism,  119,  188,  189 

Polyp,  175-178 

Polyzoa,  202 

Porcupine,  456 

Porifera,  147,  164 

Porpoises,  452 

Potato-beetle,  304 

Prairie  dog,  457 

Premolar  (teeth),  443 

Primary  vesicles  (brain),  347 

Primates,  451,  458 

Primordial  germ  cells,  32,  33,  34 

Proboscidea,  454 

Procotodoeum,  67,  280 

Proglottis  (segment),  198,  199 

Promorphology — defined,  3,  91;  illus- 
trated, 91 

Prophase,  24 

Prostomium,  229 

Protective  resemblance,  125 

Proteids,  n 

Prothorax,  272 

Protoplasm,  10;  chemical  composition 
of,  1 1 ;  physical  structure,  1 1 ;  physi- 
ology, 12,  65;  relation  to  life,  9 


INDEX 


543 


Protopodite,  267 
Protopterus,  374 
Proto-vertebrata,  311 
Protozoa,  147,  149 
Proventriculus,  280,  404 
Pseudopodia,  154 
Psychology,  469 
Ptarmigan,  418 
Pteropus,  457 

Pulp- cavity  (see  teeth),  442 
Pupa,  288  (see  metamorphosis). 
Purity  of  gametes,  494 
Pygopodes,  413 
Pyloric  caeca,  281,  316 

Quadrate  bone,  387,  442 

Quail,  416 

Quill  (see  feather). 

Rabbit,  457 

Rachis  (see  feather). 

Radial  symmetry,  93 

Radiolaria,  161 

Radula,  247 

Rail,  417 

Rana,  381 

Raptores,  419 

Rat,  455,  461 

Ratitae,  410 

Rattle-snake,  387 

Rays,  372 

Razor-clam,  253 

Receptaculum  seminis,  232,  286 

Rectrices  (see  feathers),  399 

Redia,  196 

Reduction  divisions,  32,  33,  34 

.Reference  books,  530 

Regeneration,  96 

Reintegration  of  parts,  84 

Relationships  of  animals,  485 

Remiges  (see  feathers),  399 

Renal  portal  circulation,  364 

Reproduction,  14;  of  cells,  23;  asexual, 

73;  sexual,  74,  115 
Reproduction,     organs     of — protozoa, 

156;  porifera,  169;  ccelenterata,  183; 

unsegmented  worms,  191,  194,  196; 

echinoderms,  217;  annulata,  233 ;  mol- 

lusks,  252,  253;  arthropods,  286;  ver- 


tebrates, 344,  fishes,  369;  amphibia, 
379;  reptiles,  390;  birds,  407;  mam- 
mals, 447 

Reproductive  epithelium,  51,  52,  53 

Reptiles,  142,  384;  age  of,  385 

Reptilian  birds,  409 

Resemblance,  general  and  special,  119 

Respiration,  68;  internal,  336;  ccelenter- 
ata, 181 ;  annulata,  230;  echinoderms, 
215;  mollusks,  249;  arthropods,  281; 
vertebrates,  336;  fishes,  336;  am- 
phibians, 377;  reptiles,  387;  birds, 
404,  mammals,  446 

Respiratory  tree,  215 

Retina,  82,  83,  355,  35$ 

Rhamphorhynchus,  390 

Rhinoceros,  453 

Rhinoderma,  379 

Rhizopoda,  161 

Rhodostethia,  413 

Right-left  axis  (see  symmetry). 

Rodentia,  450,  455 

Rotifers,  200,  201 

Round  worms,  199 

Rudimentary  organs,  481 

Rumen,  443 

Ruminant,  453 

Ruminant  stomach,  443 

Sacral  vertebrae,  329 
Sagittal  section,  91 
Salamander,  383 
Salmo,  368  (salmon) 
Salvelimus,  368 
Sarcolemma,  59 
Saururae,  410 
Scale  insects,  299 
Scales,  362 
Scallop,  259 
Scarabeids,  304 
Sceloporus,  393 
Schwann's  sheath,  62 
Sciurus,  456 
Scolopendra,  297 
Scorpionida,  307 
Scyphistoma,  185 
Scyphozoa,  185 
Sea-cow,  452 
Sea  cucumber,  220 


544 


INDEX 


Sea  lilies,  219 

Sea  squirts,  312 

Sea  urchins,  220 

Seals,  454 

Secretion,  17,  69 

Sections,  defined,  91 

Segment,   278  (see  metamere,  somite). 

Segmentation  (see  cleavage,  metamer- 
ism). 

Segmentation  cavity,  38,  39 

Segmented  worms  (see  annulata). 

Segregation  of  characters,  494 

Segregation  of  germ  cells,  42 

Semi-circular  canals,  354 

Seminal  vesicles,  232 

Sensation,  76,  148;  see  also  sense  organs. 

Sense  organs — protozoa,  155;  porifera, 
169;  ccelenterata,  182;  echinoder- 
mata,  217;  annulata,  231;  mollusca, 
251;  arthropoda,  283;  vertebrata, 
347;  reptiles,  388;  birds,  405;  mam- 
mals, 446 

Sensory  epithelium,  51,  52 

Sexual  cells,  30  (see  ovum,  sperm). 

Shark,  372 

Sheep,  453 

Sheepshead  (fish),  373 

Shell,  245;  structure,  247 

Ship  worm,  258 

Shrike,  429 

Shrimp,  295 

Sight,  8 1  (see  sense  organs). 

Silk  worm,  304 

Siphon,  240,  255,  256 

Siphonophora,  185,  187 

Sipunculids,  203 

Sipunculoidea,  238 

Siren,  380,  383 

Sirenia,  450,  452 

Skates,  372 

Skin,  326  (see  integument). 

Skin  senses,  353 

Skeletal  system,  72 — in  protozoa,  153; 
porifera,  168;  ccelenterates,  182; 
echinoderms,  212,  216;  annulates, 
228;  mollusks,  244;  arthropods,  279; 
vertebrates,  327;  fishes,  359;  am- 
phibia, 377;  reptiles,  386;  birds,  403; 
mammals,  442 


Skull,  362,  387,  4i9i  422 

Sloths,  451 

Slugs,  260 

Smell,  80,  353 

Smelt,  370 

Snails,  242,  246,  260 

Snakes,  393 

Snipe,  417 

Snowy  grouper,  373 

Social  instincts,  120,  287,  449,  470 

Sociology,  470 

Somite  (see  segment). 

Sow-bug,  269,  293 

Sparrows,  420 

Special  senses,  80,  353 

Species,  109-111 

Spermatids,  32,  33,  34 

Spermatocytes,  32,  33,  34 

Spermatozoon,  31 

Sphincter,  335 

Spider,  306,  308 

Spinal  cord,  351 

Spinal  nerves,  351 

Spinous  process,  328 

Spinus,  426 

Spiracle,  272 

Spirostomum,  155 

Spleen,  341 

Sponges,  147,  164  (see  porifera) . 

Spontaneous  generation,  160 

Sporocyst,  195,  196 

Sporozoa,  161 

Sports,  488 

Squamous  epithelium,  49 

Squid,  243,  262 

Starfish,  205,  211 

Statocyst,  80,  8 1,  284  (see  otocyst) 

Statoliths,  80,  8 1 

Steganopodes,  415 

Stentor,  155 

Sternum,  266,  331 

Stigmata,  281 

Stimulus,  13 

Stolon,  184 

Stomach,  335,  444 

Stomodaeum,  67,  280 

Stone  flies,  298 

Stork,  415 

Streptoneura,  260 


INDEX 


545 


Striate  muscle,  60 

Strobila,  197 

Struggle  for  existence,  103 

Struthio,  411 

Sturgeon,  372 

Sunfish,  372 

Supplementary  studies — protozoa,  163; 
porifera,  172;  ccelenterata,  189;  un- 
segmented  worms,  205;  echinoderms, 
220 ;  annulata,  238;  mollusca,  263; 
arthropods,  309;  fishes,  374;  reptiles, 
396;  birds,  436;  mammals,  463 

Supportive  tissue  (see  connective) . 

Suture,  442 

Swallow-tailed  butterfly,  305 

Sweat  glands,  326 

Swifts,  418 

Swine,  453 

Symbiosis,  122 

Symmetry,  3,  91,  371 

Sympathetic  system,  353 

Syrinx,  405,  431 

Tcenia,  197 

Tail,  361 

Tape- worn  (see  Tcenia),  198 

Tapir,  454 

Tarso-metatarsus,  404 

Tarsus,  272,  332 

Teeth,  442 

Teleostei,  372 

Telophase,  26 

Tent  caterpillars,  303 

Teredo,  258 

Tergum,  266 

Termites,  298 

Testis,  75,  232 

Tetrabranchiata,  262 

Thread- worms,  199 

Thrips,  299 

Thrushes,  427,  430 

Thymus,  341 

Thyroid,  341 

Thysanura,  298 

Tibia,  272 

Ticks,  309 

Tiger,  454 

Tissues — definition,  47;  differentiation, 

41 ;  classification,  48 
Toad,  381 


Tortoises,  392 
Tortoise-shell,  386 
Toucans,  428 
'Touch,  80 
Trachea,  337 
Tracheae,  69,  281 
Transverse  section,  91 
Tree-toads,  383 
Trematodes,  195 
Trichinella,  199 
Trichoptera,  299 
Trivium,  212 
Trochanter,  272 
Trochophore,  201,  234,  254 ; 
Troglodytidae,  430 
Trophoblast,  447,  448 
Tube-feet,  214 
Tubifex,  238 
Tunicates,  312 
Turbellaria,  193,  194 
Turdidae,  430 
Turkey,  416 
Turtles,  392 

Tympanic  membrane,  354 
Typhlosole,  230 

Ungulata,  450,  452 

Unit  characters,  492 

Unio,  239,  258 

Univalent  chromosomes,  33,  34,  35 

Unsegmented  worms,  146,  190 

Upupa,  402 

Urea,  71,  344 

Ureter,  344,  447 

Urethra,  344,  437 

Uric  acid,  71 

Urinary  bladder,  344,  447 

Urodela,  383 

Uterus,  192,  345,  447 

Vagus  (nerve),  352 

Variability,  101,  477,  487 

Varieties,  109 

Vas  deferens,  252,  286,  344 

Veins,  71 

Veliger,  254 

Venomous  snakes,  389 

Ventriculus  (stomach),  280 

Vermes,  203 

Vermiform  appendix  (caecum),  445 


546 


INDEX 


Vertebrae,  329,  330 

Vertebral  column,  328;  divisions  of,  329 

Vertebrates,  144,  312 

Villi,  335,  448 

Visceral  arches,  336 

Viviparous,  345 

Volvox,  153,  158,  161 

Vorticella,  157,  158,  161 

Vultures,  419 

Walking-stick  insect,  121,  124,  297 

Walrus,  454 

Warblers,  420 

Wasps,  306 

Water-boatman,  299 

Water-vascular  system,  213,  216 

Weasel,  454  - 

Web  (of  spiders),  305;  birds,  401 

Weevils,  304 


Whales,  452 

Whelks,  260 

Whippoorwill,  429 

White  ants,  298 

Wings — insects,  279;  birds,   400;  bats, 

457 

Wolves,  454 
Wood-lice,  299 
Wood-peckers,  424,  426 
Worms,  146,  190,  222 
Wrens,  420 

Xiphosura,  306 

Yolk,  37 ;  influence  on  cleavage,  38 

Zebra,  454 

Zoea,  287 

Zoology — scope,  2;  history  of,  6,  and 


Date  Due 


^007 

_^_ 


MAR    17 


1932 


FEE  1 


L£ L 


"  . 


MOV  '1  1QAH 


LIBRARY 

COLLEGE    OF    DENTISTRY 
UNIVERSITY   OF  CALIFORNIA 


